diff --git a/scripts/lldbinit.py b/scripts/lldbinit.py
index ff1732ab9157..3102e17a9d87 100644
--- a/scripts/lldbinit.py
+++ b/scripts/lldbinit.py
@@ -86,6 +86,7 @@ Example usage:
         "constant": "SemIR::MakeConstantId",
         "constraint": "SemIR::MakeNamedConstraintId",
         "declared_facet_type": "SemIR::MakeDeclaredFacetTypeId",
+        "default_value": "SemIR::MakeDefaultValueId",
         "entity_name": "SemIR::MakeEntityNameId",
         "function": "SemIR::MakeFunctionId",
         "generated_function": "SemIR::MakeGeneratedFunctionId",
diff --git a/toolchain/check/context.h b/toolchain/check/context.h
index 5e2b46f1092b..b47aa9a90c22 100644
--- a/toolchain/check/context.h
+++ b/toolchain/check/context.h
@@ -401,6 +401,9 @@ class Context {
   auto declared_facet_types() -> SemIR::DeclaredFacetTypeStore& {
     return sem_ir().declared_facet_types();
   }
+  auto default_values() -> SemIR::DefaultValueStore& {
+    return sem_ir().default_values();
+  }
   auto identified_facet_types() -> SemIR::IdentifiedFacetTypeStore& {
     return sem_ir().identified_facet_types();
   }
diff --git a/toolchain/check/cpp/import.cpp b/toolchain/check/cpp/import.cpp
index 0b1aa7950ad4..3e997c675cdf 100644
--- a/toolchain/check/cpp/import.cpp
+++ b/toolchain/check/cpp/import.cpp
@@ -1967,7 +1967,6 @@ static auto ImportFunction(Context& context, SemIR::LocId loc_id,
               .call_param_patterns_id =
                   function_params_insts->call_param_patterns_id,
               .call_params_id = function_params_insts->call_params_id,
-              .call_param_default_values_id = SemIR::InstBlockId::Empty,
               .call_param_ranges = function_params_insts->param_ranges,
               .return_type_inst_id = function_params_insts->return_type_inst_id,
               .return_form_inst_id = function_params_insts->return_form_inst_id,
diff --git a/toolchain/check/dump.cpp b/toolchain/check/dump.cpp
index 3a16396a1809..636241e49455 100644
--- a/toolchain/check/dump.cpp
+++ b/toolchain/check/dump.cpp
@@ -64,18 +64,24 @@ LLVM_DUMP_METHOD static auto Dump(const Context& context,
   return SemIR::Dump(context.sem_ir(), const_id);
 }
 
-LLVM_DUMP_METHOD static auto Dump(const Context& context,
-                                  SemIR::EntityNameId entity_name_id)
-    -> std::string {
-  return SemIR::Dump(context.sem_ir(), entity_name_id);
-}
-
 LLVM_DUMP_METHOD static auto Dump(
     const Context& context, SemIR::DeclaredFacetTypeId declared_facet_type_id)
     -> std::string {
   return SemIR::Dump(context.sem_ir(), declared_facet_type_id);
 }
 
+LLVM_DUMP_METHOD static auto Dump(const Context& context,
+                                  SemIR::DefaultValueId value_id)
+    -> std::string {
+  return SemIR::Dump(context.sem_ir(), value_id);
+}
+
+LLVM_DUMP_METHOD static auto Dump(const Context& context,
+                                  SemIR::EntityNameId entity_name_id)
+    -> std::string {
+  return SemIR::Dump(context.sem_ir(), entity_name_id);
+}
+
 LLVM_DUMP_METHOD static auto Dump(const Context& context,
                                   SemIR::FunctionId function_id)
     -> std::string {
diff --git a/toolchain/check/full_pattern_stack.h b/toolchain/check/full_pattern_stack.h
index ba30486e8ea3..7f6c6e080bee 100644
--- a/toolchain/check/full_pattern_stack.h
+++ b/toolchain/check/full_pattern_stack.h
@@ -81,6 +81,7 @@ class FullPatternStack {
     bind_name_stack_.PushArray();
     var_pattern_stack_.PushArray();
     next_var_index_stack_.push_back(-1);
+    unspecified_default_values_stack_.PushArray();
   }
 
   // Marks the start of a new full-pattern for a name binding declaration.
@@ -89,6 +90,7 @@ class FullPatternStack {
     bind_name_stack_.PushArray();
     var_pattern_stack_.PushArray();
     next_var_index_stack_.push_back(-1);
+    unspecified_default_values_stack_.PushArray();
   }
 
   // Marks the start of a new full-pattern for a class `var` declaration.
@@ -97,6 +99,7 @@ class FullPatternStack {
     bind_name_stack_.PushArray();
     var_pattern_stack_.PushArray();
     next_var_index_stack_.push_back(-1);
+    unspecified_default_values_stack_.PushArray();
   }
 
   // Marks the start of the current parameterized entity's implicit parameter
@@ -121,8 +124,6 @@ class FullPatternStack {
     CARBON_CHECK(kind_stack_.back() == Kind::NotInEitherParamList, "{0}",
                  kind_stack_.back());
     kind_stack_.back() = Kind::ExplicitParamList;
-    raw_default_values_stack_.PushArray();
-    converted_default_values_stack_.PushArray();
   }
 
   // Marks the end of the current parameterized entity's explicit parameter
@@ -142,17 +143,14 @@ class FullPatternStack {
   // Marks the end of checking and pattern matching for the current
   // full-pattern.
   auto PopFullPattern() -> void {
-    auto kind = kind_stack_.pop_back_val();
+    kind_stack_.pop_back();
     bind_name_stack_.PopArray();
     int index = next_var_index_stack_.pop_back_val();
     CARBON_CHECK(index < 0 || static_cast<size_t>(index) ==
                                   var_pattern_stack_.PeekArray().size(),
                  "`GetLocalVarStorage` not called for all var patterns");
     var_pattern_stack_.PopArray();
-    if (kind == Kind::ExplicitParamList) {
-      raw_default_values_stack_.PopArray();
-      converted_default_values_stack_.PopArray();
-    }
+    unspecified_default_values_stack_.PopArray();
   }
 
   // Records that `name_id` was introduced by the current full-pattern.
@@ -207,51 +205,18 @@ class FullPatternStack {
                  kind_stack_.size());
   }
 
-  // Adds the inst id for a default value for any subpattern in the
-  // full-pattern. We store these before the type of the pattern with this
-  // default value is known. Returns the index of the added instruction
-  // as a `DefaultValueId`. Note default values are only supported for
-  // explicit parameter lists.
-  auto AddRawDefaultValue(SemIR::InstId inst_id) -> SemIR::DefaultValueId {
-    CARBON_CHECK(kind_stack_.back() == Kind::ExplicitParamList);
-    return AddDefaultValue(inst_id, raw_default_values_stack_);
+  // Adds an unspecified pattern default value to the array at the top of the
+  // full pattern stack. We track these for possible later use in diagnostics.
+  auto AddUnspecifiedDefaultValue(SemIR::InstId inst_id) -> void {
+    unspecified_default_values_stack_.AppendToTop(inst_id);
   }
 
-  // Adds the inst id for a default value after conversion to the pattern type.
-  // Returns the index of the added instruction as a `DefaultValueId`.
-  auto AddConvertedDefaultValue(SemIR::InstId inst_id)
-      -> SemIR::DefaultValueId {
-    return AddDefaultValue(inst_id, converted_default_values_stack_);
-  }
-
-  // Returns a reference to the array of raw default value inst ids at the top
-  // of the stack. Note default values are only supported for explicit parameter
-  // lists.
-  auto GetRawDefaultValues() -> llvm::ArrayRef<SemIR::InstId> {
-    return raw_default_values_stack_.empty()
-               ? llvm::ArrayRef<SemIR::InstId>()
-               : raw_default_values_stack_.PeekArray();
-  }
-
-  // Returns a reference to the array of type-converted default value inst ids
-  // at the top of the stack.
-  auto GetConvertedDefaultValues() -> llvm::ArrayRef<SemIR::InstId> {
-    return converted_default_values_stack_.empty()
-               ? llvm::ArrayRef<SemIR::InstId>()
-               : converted_default_values_stack_.PeekArray();
+  // Returns the unspecified default values array at the top of the stack.
+  auto GetUnspecifiedDefaultValues() -> llvm::ArrayRef<SemIR::InstId> {
+    return unspecified_default_values_stack_.PeekArray();
   }
 
  private:
-  // TODO: move default value InstIds to a value store and remove them from the
-  // pattern stack.
-  auto AddDefaultValue(SemIR::InstId inst_id, ArrayStack<SemIR::InstId>& stack)
-      -> SemIR::DefaultValueId {
-    auto index =
-        SemIR::DefaultValueId(static_cast<int32_t>(stack.PeekArray().size()));
-    stack.AppendToTop(inst_id);
-    return index;
-  }
-
   LexicalLookup* lookup_;
 
   // The stack of pending full-patterns is organized as a struct of arrays, with
@@ -283,16 +248,9 @@ class FullPatternStack {
   // of that frame are not ready for consumption.
   llvm::SmallVector<int> next_var_index_stack_;
 
-  // The stack of instructions specifying default values for subpatterns
-  // within this full-pattern. These instructions are as they are written by
-  // the developer, with no type conversions applied. They are indexed by
-  // `DefaultValueId` values.
-  ArrayStack<SemIR::InstId> raw_default_values_stack_;
-
-  // The stack of instructions for default values after the conversions to the
-  // type of the pattern have been applied. They are indexed by `DefaultValueId`
-  // values.
-  ArrayStack<SemIR::InstId> converted_default_values_stack_;
+  // For each full pattern we maintain a list of the InstIds of any
+  // unspecified default values, for use in diagnostics.
+  ArrayStack<SemIR::InstId> unspecified_default_values_stack_;
 };
 
 }  // namespace Carbon::Check
diff --git a/toolchain/check/function.cpp b/toolchain/check/function.cpp
index b5229c9af9c1..b71c723e0420 100644
--- a/toolchain/check/function.cpp
+++ b/toolchain/check/function.cpp
@@ -91,7 +91,6 @@ struct FunctionSignatureInsts {
   SemIR::InstBlockId param_patterns_id = SemIR::InstBlockId::None;
   SemIR::InstBlockId call_param_patterns_id = SemIR::InstBlockId::None;
   SemIR::InstBlockId call_params_id = SemIR::InstBlockId::None;
-  SemIR::InstBlockId call_param_default_values_id = SemIR::InstBlockId::Empty;
   SemIR::Function::CallParamIndexRanges call_param_ranges =
       SemIR::Function::CallParamIndexRanges::Empty;
   SemIR::TypeInstId return_type_inst_id = SemIR::TypeInstId::None;
@@ -191,8 +190,6 @@ auto MakeGeneratedFunctionDecl(Context& context, SemIR::LocId loc_id,
           {
               .call_param_patterns_id = insts.call_param_patterns_id,
               .call_params_id = insts.call_params_id,
-              .call_param_default_values_id =
-                  insts.call_param_default_values_id,
               .call_param_ranges = insts.call_param_ranges,
               .return_type_inst_id = insts.return_type_inst_id,
               .return_form_inst_id = insts.return_form_inst_id,
@@ -305,62 +302,6 @@ static auto CheckFunctionEvaluationModeMatches(
   return false;
 }
 
-// Checks that if `new_id` has a specified value, it has the same value as
-// specified by `prev_id`. If `diagnose` is true this will issue a diagnostic
-// if it detects a difference. Returns true if the values are the same or
-// `new_id` is unspecified.
-//
-// Note: this function is only called on the function's first owning
-// declaration, as that is the declaration with this requirement.
-static auto CheckDefaultValueIsSame(Context& context, SemIR::InstId new_id,
-                                    SemIR::InstId prev_id, bool diagnose)
-    -> bool {
-  CARBON_CHECK(!context.insts().Is<SemIR::UnspecifiedValue>(prev_id));
-  if (!context.insts().Is<SemIR::UnspecifiedValue>(new_id)) {
-    auto new_constant_id = context.constant_values().Get(new_id);
-    auto prev_constant_id = context.constant_values().Get(prev_id);
-    if (new_constant_id != prev_constant_id) {
-      if (diagnose) {
-        CARBON_DIAGNOSTIC(
-            PatternDefaultValueDiffers, Error,
-            "default value of {0} differs from the previously declared default "
-            "value of {1}",
-            InstIdAsConstant, InstIdAsConstant);
-        CARBON_DIAGNOSTIC(PatternDefaultValueDiffersNote, Note,
-                          "different previous declaration here");
-        context.emitter()
-            .Build(new_id, PatternDefaultValueDiffers, new_id, prev_id)
-            .Note(prev_id, PatternDefaultValueDiffersNote)
-            .Emit();
-      }
-      return false;
-    }
-  }
-
-  return true;
-}
-
-// Checks every parameter in `prev_function` and `new_function`, that if they
-// both specify a default value those values are identical. If `diagnose` is
-// true, issues diagnostics when that condition is violated. Returns true if
-// every parameter met the condition.
-static auto CheckDefaultValueConsistency(Context& context,
-                                         const SemIR::Function& new_function,
-                                         const SemIR::Function& prev_function,
-                                         bool diagnose) -> bool {
-  auto new_default_value_ids = context.inst_blocks().GetOrEmpty(
-      new_function.call_param_default_values_id);
-  auto prev_default_value_ids = context.inst_blocks().GetOrEmpty(
-      prev_function.call_param_default_values_id);
-
-  return llvm::all_of(
-      llvm::zip_equal(new_default_value_ids, prev_default_value_ids),
-      [&context, diagnose](auto id_pair) -> bool {
-        auto [new_id, prev_id] = id_pair;
-        return CheckDefaultValueIsSame(context, new_id, prev_id, diagnose);
-      });
-}
-
 auto CheckFunctionTypeMatches(Context& context,
                               const SemIR::Function& new_function,
                               const SemIR::Function& prev_function,
@@ -379,10 +320,6 @@ auto CheckFunctionTypeMatches(Context& context,
                                           diagnose)) {
     return false;
   }
-  if (!CheckDefaultValueConsistency(context, new_function, prev_function,
-                                    diagnose)) {
-    return false;
-  }
   return true;
 }
 
diff --git a/toolchain/check/global_init.cpp b/toolchain/check/global_init.cpp
index f95ccfef9d45..561330520441 100644
--- a/toolchain/check/global_init.cpp
+++ b/toolchain/check/global_init.cpp
@@ -51,7 +51,6 @@ auto GlobalInit::Finalize() -> void {
         .first_owning_decl_id = SemIR::InstId::None},
        {.call_param_patterns_id = SemIR::InstBlockId::Empty,
         .call_params_id = SemIR::InstBlockId::Empty,
-        .call_param_default_values_id = SemIR::InstBlockId::Empty,
         .call_param_ranges = SemIR::Function::CallParamIndexRanges::Empty,
         .return_type_inst_id = SemIR::TypeInstId::None,
         .return_form_inst_id = SemIR::InstId::None,
diff --git a/toolchain/check/handle_function.cpp b/toolchain/check/handle_function.cpp
index 0eb5af6f1464..20efd3fdc83f 100644
--- a/toolchain/check/handle_function.cpp
+++ b/toolchain/check/handle_function.cpp
@@ -373,18 +373,13 @@ static auto DiagnosePositionalParams(Context& context,
   function_info.param_patterns_id = SemIR::InstBlockId::Empty;
 }
 
-// Diagnoses that the default values for function parameters have been
+// Diagnoses any default values for function parameters that have not been
 // completely specified, which is a requirement on the first owning declaration
 // of a function.
-static auto CheckDefaultValuesCompletelySpecified(
-    Context& context, SemIR::Function& function_info) -> void {
-  auto unspecified_value_ids = llvm::make_filter_range(
-      context.inst_blocks().GetOrEmpty(
-          function_info.call_param_default_values_id),
-      [&context](auto inst_id) {
-        return context.insts().Is<SemIR::UnspecifiedValue>(inst_id);
-      });
-  for (auto inst_id : unspecified_value_ids) {
+static auto DiagnoseDefaultValuesNotSpecified(
+    Context& context, llvm::ArrayRef<SemIR::InstId> unspecified_inst_ids)
+    -> void {
+  for (auto inst_id : unspecified_inst_ids) {
     CARBON_DIAGNOSTIC(PatternDefaultValueNotSpecified, Error,
                       "found unspecified default parameter value in the "
                       "function's first owning declaration");
@@ -455,22 +450,21 @@ static auto BuildFunctionDecl(Context& context,
 
   // Build the function entity. This will be merged into an existing function if
   // there is one, or otherwise added to the function store.
-  auto function_info = SemIR::Function{
-      name_context.MakeEntityWithParamsBase(name, decl_id, is_extern,
-                                            introducer.extern_library),
-      {
-          .call_param_patterns_id = name.call_param_patterns_id,
-          .call_params_id = name.call_params_id,
-          .call_param_default_values_id = name.call_param_default_values_id,
-          .call_param_ranges = name.param_ranges,
-          .return_type_inst_id = return_type_inst_id,
-          .return_form_inst_id = return_form_inst_id,
-          .return_pattern_id = return_pattern_id,
-          .virtual_modifier = virtual_modifier,
-          .evaluation_mode = evaluation_mode,
-          .interface_modifier = interface_modifier,
-          .self_param_id = self_param_id,
-      }};
+  auto function_info =
+      SemIR::Function{name_context.MakeEntityWithParamsBase(
+                          name, decl_id, is_extern, introducer.extern_library),
+                      {
+                          .call_param_patterns_id = name.call_param_patterns_id,
+                          .call_params_id = name.call_params_id,
+                          .call_param_ranges = name.param_ranges,
+                          .return_type_inst_id = return_type_inst_id,
+                          .return_form_inst_id = return_form_inst_id,
+                          .return_pattern_id = return_pattern_id,
+                          .virtual_modifier = virtual_modifier,
+                          .evaluation_mode = evaluation_mode,
+                          .interface_modifier = interface_modifier,
+                          .self_param_id = self_param_id,
+                      }};
   if (is_definition) {
     function_info.definition_id = decl_id;
   }
@@ -478,7 +472,8 @@ static auto BuildFunctionDecl(Context& context,
   DiagnosePositionalParams(context, function_info);
   if (name_context.state != DeclNameStack::NameContext::State::Poisoned &&
       !name_context.prev_inst_id().has_value()) {
-    CheckDefaultValuesCompletelySpecified(context, function_info);
+    DiagnoseDefaultValuesNotSpecified(
+        context, context.inst_blocks().Get(name.unspecified_values_block_id));
   }
 
   TryMergeRedecl(
diff --git a/toolchain/check/handle_impl.cpp b/toolchain/check/handle_impl.cpp
index 3b2d891f9246..eb5e0c7c559c 100644
--- a/toolchain/check/handle_impl.cpp
+++ b/toolchain/check/handle_impl.cpp
@@ -200,9 +200,9 @@ static auto PopImplIntroducerAndParamsAsNameComponent(
           .param_patterns_id = SemIR::InstBlockId::None,
           .call_param_patterns_id = SemIR::InstBlockId::None,
           .call_params_id = SemIR::InstBlockId::None,
-          .call_param_default_values_id = SemIR::InstBlockId::Empty,
           .param_ranges = SemIR::Function::CallParamIndexRanges::Empty,
-          .pattern_block_id = pattern_block_id};
+          .pattern_block_id = pattern_block_id,
+          .unspecified_values_block_id = SemIR::InstBlockId::Empty};
 }
 
 // Build an ImplDecl describing the signature of an impl. This handles the
diff --git a/toolchain/check/handle_pattern_list.cpp b/toolchain/check/handle_pattern_list.cpp
index 1f61a6099300..fa1eaf5a043c 100644
--- a/toolchain/check/handle_pattern_list.cpp
+++ b/toolchain/check/handle_pattern_list.cpp
@@ -154,11 +154,15 @@ auto HandleParseNode(Context& context, Parse::PatternListCommaId /*node_id*/)
 
 auto HandleParseNode(Context& context, Parse::DefaultValueUnspecifiedId node_id)
     -> bool {
-  context.node_stack().Push(
-      node_id, AddInst<SemIR::UnspecifiedValue>(
-                   context, node_id,
-                   {.type_id = GetSingletonType(
-                        context, SemIR::UnspecifiedValueType::TypeInstId)}));
+  auto inst_id = AddInst<SemIR::UnspecifiedValue>(
+      context, node_id,
+      {.type_id =
+           GetSingletonType(context, SemIR::UnspecifiedValueType::TypeInstId)});
+
+  // Add the unspecified default value for later diagnostics checks.
+  context.full_pattern_stack().AddUnspecifiedDefaultValue(inst_id);
+
+  context.node_stack().Push(node_id, inst_id);
   return true;
 }
 
@@ -194,11 +198,13 @@ auto HandleParseNode(Context& context, Parse::DefaultValuePatternId node_id)
     return false;
   }
 
-  // Add the value to the default values array in the full pattern stack, for
-  // recovery later in the NameComponent. We store the raw value here for
+  // Add the value to the default values store. We store the raw value here for
   // conversion during pattern matching once the type of the pattern is known.
-  auto default_value_id =
-      context.full_pattern_stack().AddRawDefaultValue(expr_inst_id);
+  auto default_value_id = context.default_values().Add(
+      {.raw_id = expr_inst_id,
+       .value_id = SemIR::InstId::None,
+       .is_unspecified =
+           context.insts().Is<SemIR::UnspecifiedValue>(expr_inst_id)});
 
   // Next on the node stack should be the pattern for which this default was
   // specified. We pop that so we can issue the DefaultValuePattern in its
diff --git a/toolchain/check/import_ref.cpp b/toolchain/check/import_ref.cpp
index 9393efed26fc..47e6b7398ad0 100644
--- a/toolchain/check/import_ref.cpp
+++ b/toolchain/check/import_ref.cpp
@@ -179,6 +179,9 @@ class ImportContext {
   auto import_constant_values() -> const SemIR::ConstantValueStore& {
     return import_ir().constant_values();
   }
+  auto import_default_values() -> const SemIR::DefaultValueStore& {
+    return import_ir().default_values();
+  }
   auto import_entity_names() -> const SemIR::EntityNameStore& {
     return import_ir().entity_names();
   }
@@ -267,6 +270,9 @@ class ImportContext {
   auto local_constant_values() -> SemIR::ConstantValueStore& {
     return local_ir().constant_values();
   }
+  auto local_default_values() -> SemIR::DefaultValueStore& {
+    return local_ir().default_values();
+  }
   auto local_entity_names() -> SemIR::EntityNameStore& {
     return local_ir().entity_names();
   }
@@ -2306,6 +2312,12 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::DefaultValuePattern inst)
     -> ResolveResult {
   auto subpattern = GetLocalImportRefInfo(resolver, inst.subpattern_id);
+  const auto& import_default_value =
+      resolver.import_default_values().Get(inst.default_value_id);
+  // We import the first owning declaration of a function, which must always
+  // have default values completely specified.
+  CARBON_CHECK(!import_default_value.is_unspecified);
+  auto value = GetLocalImportRefInfo(resolver, import_default_value.value_id);
   if (resolver.HasNewWork()) {
     return ResolveResult::Retry();
   }
@@ -2316,7 +2328,10 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
           .type_id = resolver.local_types().GetTypeIdForTypeConstantId(
               subpattern.local_type_const_id),
           .subpattern_id = AddLoadedImportRef(resolver, subpattern),
-          .default_value_id = inst.default_value_id,
+          .default_value_id = resolver.local_default_values().Add(
+              {.raw_id = SemIR::InstId::None,
+               .value_id = AddLoadedImportRef(resolver, value),
+               .is_unspecified = false}),
       });
 }
 
@@ -2425,7 +2440,6 @@ static auto ImportFunctionDecl(
       {GetIncompleteLocalEntityBase(context, function_decl_id, import_function),
        {.call_param_patterns_id = SemIR::InstBlockId::None,
         .call_params_id = SemIR::InstBlockId::None,
-        .call_param_default_values_id = SemIR::InstBlockId::Empty,
         .call_param_ranges = import_function.call_param_ranges,
         .return_type_inst_id = SemIR::TypeInstId::None,
         .return_form_inst_id = SemIR::InstId::None,
@@ -2571,8 +2585,6 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
   auto call_param_patterns = GetLocalBlockImportRefInfo(
       resolver, import_function.call_param_patterns_id);
-  auto call_param_default_values = GetLocalBlockImportRefInfo(
-      resolver, import_function.call_param_default_values_id);
   auto return_type_const_id = SemIR::ConstantId::None;
   if (import_function.return_type_inst_id.has_value()) {
     return_type_const_id =
@@ -2627,10 +2639,6 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   // Add the function declaration.
   new_function.call_param_patterns_id =
       AddLoadedImportRefBlock(resolver, call_param_patterns);
-  if (call_param_default_values.has_value()) {
-    new_function.call_param_default_values_id =
-        AddLoadedImportRefBlock(resolver, *call_param_default_values);
-  }
   new_function.parent_scope_id = parent_scope_id;
   new_function.implicit_param_patterns_id =
       AddLoadedImportRefBlock(resolver, implicit_param_patterns);
diff --git a/toolchain/check/merge.cpp b/toolchain/check/merge.cpp
index 296cb0a6beeb..90ed33f7d37e 100644
--- a/toolchain/check/merge.cpp
+++ b/toolchain/check/merge.cpp
@@ -339,14 +339,30 @@ static auto CheckRedeclParam(Context& context, bool is_implicit_param,
         auto prev_default_value_pattern =
             prev_param_pattern.As<SemIR::DefaultValuePattern>();
 
+        // If the new pattern specified a default value, it must match the
+        // previously declared default value.
+        const auto& new_default_value = context.default_values().Get(
+            new_default_value_pattern.default_value_id);
+
+        if (!new_default_value.is_unspecified) {
+          const auto& prev_default_value = context.default_values().Get(
+              prev_default_value_pattern.default_value_id);
+          // We require first owning declaration to always specify a default
+          // value.
+          CARBON_CHECK(!prev_default_value.is_unspecified);
+          auto new_constant_id =
+              context.constant_values().Get(new_default_value.value_id);
+          auto prev_constant_id =
+              context.constant_values().Get(prev_default_value.value_id);
+          if (new_constant_id != prev_constant_id) {
+            emit_general_diagnostic();
+            return false;
+          }
+        }
+
         pattern_stack.push_back(
             {.prev_id = prev_default_value_pattern.subpattern_id,
              .new_id = new_default_value_pattern.subpattern_id});
-
-        // The node kind comparison should catch this on the mismatched patterns
-        // prior to this, so the indices should never mismatch.
-        CARBON_CHECK(prev_default_value_pattern.default_value_id.index ==
-                     new_default_value_pattern.default_value_id.index);
         break;
       }
       default: {
diff --git a/toolchain/check/name_component.cpp b/toolchain/check/name_component.cpp
index 89190863e256..3a431beb058f 100644
--- a/toolchain/check/name_component.cpp
+++ b/toolchain/check/name_component.cpp
@@ -45,10 +45,10 @@ auto PopNameComponent(Context& context, SemIR::InstId return_pattern_id)
   }
 
   auto call_param_patterns_id = SemIR::InstBlockId::None;
-  auto call_param_default_values_id = SemIR::InstBlockId::Empty;
   auto call_params_id = SemIR::InstBlockId::None;
   auto param_ranges = SemIR::Function::CallParamIndexRanges::Empty;
   auto pattern_block_id = SemIR::InstBlockId::None;
+  auto unspecified_values_block_id = SemIR::InstBlockId::Empty;
   if (param_patterns_id->has_value() ||
       implicit_param_patterns_id->has_value() ||
       return_pattern_id.has_value()) {
@@ -57,11 +57,8 @@ auto PopNameComponent(Context& context, SemIR::InstId return_pattern_id)
     call_param_patterns_id = results.call_param_patterns_id;
     call_params_id = results.call_params_id;
     param_ranges = results.param_ranges;
-    CARBON_CHECK(
-        context.full_pattern_stack().GetRawDefaultValues().size() ==
-        context.full_pattern_stack().GetConvertedDefaultValues().size());
-    call_param_default_values_id = context.inst_blocks().Add(
-        context.full_pattern_stack().GetConvertedDefaultValues());
+    unspecified_values_block_id = context.inst_blocks().Add(
+        context.full_pattern_stack().GetUnspecifiedDefaultValues());
     pattern_block_id = context.pattern_block_stack().Pop();
     context.full_pattern_stack().PopFullPattern();
   }
@@ -69,21 +66,19 @@ auto PopNameComponent(Context& context, SemIR::InstId return_pattern_id)
   auto [name_loc_id, name_id] =
       context.node_stack().PopWithNodeId<Parse::NodeCategory::NonExprName>();
 
-  return {
-      .name_loc_id = name_loc_id,
-      .name_id = name_id,
-      .first_param_node_id = first_param_node_id,
-      .last_param_node_id = last_param_node_id,
-      .implicit_params_loc_id = implicit_params_node_id,
-      .implicit_param_patterns_id = *implicit_param_patterns_id,
-      .params_loc_id = params_node_id,
-      .param_patterns_id = *param_patterns_id,
-      .call_param_patterns_id = call_param_patterns_id,
-      .call_params_id = call_params_id,
-      .call_param_default_values_id = call_param_default_values_id,
-      .param_ranges = param_ranges,
-      .pattern_block_id = pattern_block_id,
-  };
+  return {.name_loc_id = name_loc_id,
+          .name_id = name_id,
+          .first_param_node_id = first_param_node_id,
+          .last_param_node_id = last_param_node_id,
+          .implicit_params_loc_id = implicit_params_node_id,
+          .implicit_param_patterns_id = *implicit_param_patterns_id,
+          .params_loc_id = params_node_id,
+          .param_patterns_id = *param_patterns_id,
+          .call_param_patterns_id = call_param_patterns_id,
+          .call_params_id = call_params_id,
+          .param_ranges = param_ranges,
+          .pattern_block_id = pattern_block_id,
+          .unspecified_values_block_id = unspecified_values_block_id};
 }
 
 // Pop the name of a declaration from the node stack, and diagnose if it has
diff --git a/toolchain/check/name_component.h b/toolchain/check/name_component.h
index 745bd70b5434..8ce99b9595e2 100644
--- a/toolchain/check/name_component.h
+++ b/toolchain/check/name_component.h
@@ -40,12 +40,13 @@ struct NameComponent {
   // SemIR::EntityWithParamsBase).
   SemIR::InstBlockId call_param_patterns_id;
   SemIR::InstBlockId call_params_id;
-  // The pattern default values as extracted from the parameter list.
-  SemIR::InstBlockId call_param_default_values_id;
   SemIR::Function::CallParamIndexRanges param_ranges;
 
   // The pattern block.
   SemIR::InstBlockId pattern_block_id;
+
+  // The `UnspecifiedValue` insts from the parameter default values, if any.
+  SemIR::InstBlockId unspecified_values_block_id;
 };
 
 // Pops a name component from the node stack (and pattern block stack, if it has
diff --git a/toolchain/check/pattern_match.cpp b/toolchain/check/pattern_match.cpp
index 7c1218a857b1..0eb3d52ff06b 100644
--- a/toolchain/check/pattern_match.cpp
+++ b/toolchain/check/pattern_match.cpp
@@ -962,21 +962,13 @@ auto MatchContext::DoPostWork(State state,
 
   // If a constant was specified, we should be able to convert it into the
   // type of the parameter.
-  auto raw_default_value_inst_id =
-      context_.full_pattern_stack()
-          .GetRawDefaultValues()[default_value_pattern.default_value_id.index];
-  auto converted_inst_id =
-      context_.insts().Is<SemIR::UnspecifiedValue>(raw_default_value_inst_id)
-          ? raw_default_value_inst_id
-          : ConvertToValueOfType(context_,
-                                 SemIR::LocId(raw_default_value_inst_id),
-                                 raw_default_value_inst_id, param_type_id);
-
-  // The index of the converted value should be the same as the raw value.
-  auto converted_default_id =
-      context_.full_pattern_stack().AddConvertedDefaultValue(converted_inst_id);
-  CARBON_CHECK(converted_default_id.index ==
-               default_value_pattern.default_value_id.index);
+  auto& default_value =
+      context_.default_values().Get(default_value_pattern.default_value_id);
+  if (!default_value.is_unspecified) {
+    default_value.value_id =
+        ConvertToValueOfType(context_, SemIR::LocId(default_value.raw_id),
+                             default_value.raw_id, param_type_id);
+  }
 
   results_stack_.PopArray();
 
diff --git a/toolchain/check/testdata/basics/raw_sem_ir/builtins.carbon b/toolchain/check/testdata/basics/raw_sem_ir/builtins.carbon
index 892d173c8806..beb1b5cd1c23 100644
--- a/toolchain/check/testdata/basics/raw_sem_ir/builtins.carbon
+++ b/toolchain/check/testdata/basics/raw_sem_ir/builtins.carbon
@@ -21,6 +21,7 @@
 // CHECK:STDOUT:   import_ir_insts: {}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {}}
 // CHECK:STDOUT:   entity_names:    {}
diff --git a/toolchain/check/testdata/basics/raw_sem_ir/bundle.carbon b/toolchain/check/testdata/basics/raw_sem_ir/bundle.carbon
index eb7d6619ff86..3bd252849dd1 100644
--- a/toolchain/check/testdata/basics/raw_sem_ir/bundle.carbon
+++ b/toolchain/check/testdata/basics/raw_sem_ir/bundle.carbon
@@ -30,6 +30,7 @@ fn F(generic Form: Core.Form) ->? Form;
 // CHECK:STDOUT:     import_ir_inst0: {ir_id: import_ir70000003, inst_id: inst50000023}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name(Core): inst70000012, name0: inst70000033}}
 // CHECK:STDOUT:     name_scope70000001: {inst: inst70000012, parent_scope: name_scope0, has_error: false, extended_scopes: [], names: {name1: inst70000017}}
diff --git a/toolchain/check/testdata/basics/raw_sem_ir/cpp_interop.carbon b/toolchain/check/testdata/basics/raw_sem_ir/cpp_interop.carbon
index 3f3865bf078c..560243e7b819 100644
--- a/toolchain/check/testdata/basics/raw_sem_ir/cpp_interop.carbon
+++ b/toolchain/check/testdata/basics/raw_sem_ir/cpp_interop.carbon
@@ -65,6 +65,7 @@ fn G(x: Cpp.X) {
 // CHECK:STDOUT:   clang_decl_signatures:
 // CHECK:STDOUT:     clang_decl_signature_id50000000: {kind: normal, num_params: 0}
 // CHECK:STDOUT:     clang_decl_signature_id50000001: {kind: normal, num_params: 1, modes: [value]}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name(Cpp): inst50000012, name0: inst5000001F}}
 // CHECK:STDOUT:     name_scope50000001: {inst: inst50000012, parent_scope: name_scope0, has_error: false, extended_scopes: [], names: {name2: inst50000014, name3: inst5000002D, name4: inst50000055}}
diff --git a/toolchain/check/testdata/basics/raw_sem_ir/multifile.carbon b/toolchain/check/testdata/basics/raw_sem_ir/multifile.carbon
index b203fc96791d..a2d6c3114018 100644
--- a/toolchain/check/testdata/basics/raw_sem_ir/multifile.carbon
+++ b/toolchain/check/testdata/basics/raw_sem_ir/multifile.carbon
@@ -38,6 +38,7 @@ fn B() {
 // CHECK:STDOUT:   import_ir_insts: {}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name0: inst50000011}}
 // CHECK:STDOUT:   entity_names:    {}
@@ -142,6 +143,7 @@ fn B() {
 // CHECK:STDOUT:     import_ir_inst1: {ir_id: import_ir70000002, inst_id: inst50000011}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name1: inst70000012, name0: inst70000013}}
 // CHECK:STDOUT:     name_scope70000001: {inst: inst70000012, parent_scope: name_scope0, has_error: false, extended_scopes: [], names: {name1: inst70000018}}
diff --git a/toolchain/check/testdata/basics/raw_sem_ir/multifile_with_textual_ir.carbon b/toolchain/check/testdata/basics/raw_sem_ir/multifile_with_textual_ir.carbon
index 7141168657b4..07f44b43f0c9 100644
--- a/toolchain/check/testdata/basics/raw_sem_ir/multifile_with_textual_ir.carbon
+++ b/toolchain/check/testdata/basics/raw_sem_ir/multifile_with_textual_ir.carbon
@@ -38,6 +38,7 @@ fn B() {
 // CHECK:STDOUT:   import_ir_insts: {}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name0: inst50000011}}
 // CHECK:STDOUT:   entity_names:    {}
@@ -161,6 +162,7 @@ fn B() {
 // CHECK:STDOUT:     import_ir_inst1: {ir_id: import_ir70000002, inst_id: inst50000011}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name1: inst70000012, name0: inst70000013}}
 // CHECK:STDOUT:     name_scope70000001: {inst: inst70000012, parent_scope: name_scope0, has_error: false, extended_scopes: [], names: {name1: inst70000018}}
diff --git a/toolchain/check/testdata/basics/raw_sem_ir/non_core_interfaces.carbon b/toolchain/check/testdata/basics/raw_sem_ir/non_core_interfaces.carbon
index 69d46296a5b7..97d12f5359c3 100644
--- a/toolchain/check/testdata/basics/raw_sem_ir/non_core_interfaces.carbon
+++ b/toolchain/check/testdata/basics/raw_sem_ir/non_core_interfaces.carbon
@@ -47,6 +47,7 @@ fn UseLocalCopy[T: Copy](_: T.T1, _: T.T2) {}
 // CHECK:STDOUT:   import_ir_insts: {}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name1: inst60000011}}
 // CHECK:STDOUT:     name_scope60000001: {inst: inst60000011, parent_scope: name_scope0, has_error: false, extended_scopes: [], names: {name(SelfType): inst60000013}}
@@ -211,6 +212,7 @@ fn UseLocalCopy[T: Copy](_: T.T1, _: T.T2) {}
 // CHECK:STDOUT:     import_ir_instC: {ir_id: import_ir50000002, inst_id: inst6000001D}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name0: inst50000012, name1: inst50000013, name4: inst5000006B, name7: inst500000AA}}
 // CHECK:STDOUT:     name_scope50000001: {inst: inst50000012, parent_scope: name_scope0, has_error: false, extended_scopes: [], names: {name6: inst50000026}}
diff --git a/toolchain/check/testdata/basics/raw_sem_ir/one_file.carbon b/toolchain/check/testdata/basics/raw_sem_ir/one_file.carbon
index 702281783fd5..4678f3a10849 100644
--- a/toolchain/check/testdata/basics/raw_sem_ir/one_file.carbon
+++ b/toolchain/check/testdata/basics/raw_sem_ir/one_file.carbon
@@ -339,6 +339,7 @@ fn Foo[T: type](p: T*) -> (T*, ()) {
 // CHECK:STDOUT:     import_ir_inst129: {ir_id: import_ir78000004, inst_id: inst700002D4}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name(Core): inst78000012, name0: inst78000049}}
 // CHECK:STDOUT:     name_scope78000001: {inst: inst78000012, parent_scope: name_scope0, has_error: false, extended_scopes: [], names: {name3: inst78000059}}
diff --git a/toolchain/check/testdata/basics/raw_sem_ir/one_file_with_textual_ir.carbon b/toolchain/check/testdata/basics/raw_sem_ir/one_file_with_textual_ir.carbon
index 1c9543bb0060..74a9f94e86fb 100644
--- a/toolchain/check/testdata/basics/raw_sem_ir/one_file_with_textual_ir.carbon
+++ b/toolchain/check/testdata/basics/raw_sem_ir/one_file_with_textual_ir.carbon
@@ -29,6 +29,7 @@ fn Foo(n: ()) -> ((), ()) {
 // CHECK:STDOUT:   import_ir_insts: {}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {name0: inst5000002F}}
 // CHECK:STDOUT:   entity_names:
diff --git a/toolchain/check/testdata/function/declaration/default_values.carbon b/toolchain/check/testdata/function/declaration/default_values.carbon
index 6d653dc66839..4dc24c9e4456 100644
--- a/toolchain/check/testdata/function/declaration/default_values.carbon
+++ b/toolchain/check/testdata/function/declaration/default_values.carbon
@@ -170,7 +170,7 @@ fn F(x: C = {});
 // CHECK:STDOUT:   %a.patt: %pattern_type.d8d = wrapper_binding_pattern a, %a.param_patt [concrete]
 // CHECK:STDOUT:   %empty_struct: %empty_struct_type = struct_value () [concrete]
 // CHECK:STDOUT:   %C.val: %C = struct_value () [concrete]
-// CHECK:STDOUT:   %.68d: %pattern_type.d8d = default_value_pattern %a.patt, index: 0 [concrete]
+// CHECK:STDOUT:   %.e44: %pattern_type.d8d = default_value_pattern %a.patt, @F.%.loc10_16.7 [concrete]
 // CHECK:STDOUT:   %C.as.ImplicitAs.impl.Convert.bound: <bound method> = bound_method %C.val, %C.as.ImplicitAs.impl.Convert [concrete]
 // CHECK:STDOUT:   %.115: ref %C = temporary invalid, %C.val [concrete]
 // CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
@@ -181,7 +181,7 @@ fn F(x: C = {});
 // CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
 // CHECK:STDOUT:     %a.param_patt: %pattern_type.d8d = value_param_pattern [concrete = constants.%a.param_patt]
 // CHECK:STDOUT:     %a.patt: %pattern_type.d8d = wrapper_binding_pattern a, %a.param_patt [concrete = constants.%a.patt]
-// CHECK:STDOUT:     %.loc10_11: %pattern_type.d8d = default_value_pattern %a.patt, index: 0 [concrete = constants.%.68d]
+// CHECK:STDOUT:     %.loc10_11: %pattern_type.d8d = default_value_pattern %a.patt, %.loc10_16.7 [concrete = constants.%.e44]
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %.loc10_14.1: %empty_struct_type = struct_literal () [concrete = constants.%empty_struct]
 // CHECK:STDOUT:     %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C]
@@ -203,10 +203,7 @@ fn F(x: C = {});
 // CHECK:STDOUT:   }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @F(%a.param: %D) {
-// CHECK:STDOUT: !default_values:
-// CHECK:STDOUT:   %.loc10_16.7: %D = acquire_value %.loc10_16.6
-// CHECK:STDOUT: }
+// CHECK:STDOUT: fn @F(%a.param: %D);
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- nested.carbon
 // CHECK:STDOUT:
@@ -240,7 +237,7 @@ fn F(x: C = {});
 // CHECK:STDOUT:   %tuple.56c: %tuple.type.6ca = tuple_value (%empty_struct, %tuple.9a3) [concrete]
 // CHECK:STDOUT:   %tuple.type.bcdf: type = tuple_type (%empty_struct_type, %tuple.type.6ca) [concrete]
 // CHECK:STDOUT:   %tuple.4f4: %tuple.type.bcdf = tuple_value (%empty_struct, %tuple.56c) [concrete]
-// CHECK:STDOUT:   %.b6d: %pattern_type.f5f = default_value_pattern %.341, index: 0 [concrete]
+// CHECK:STDOUT:   %.391: %pattern_type.f5f = default_value_pattern %.341, @F.%.loc6_62.6 [concrete]
 // CHECK:STDOUT:   %C.val: %C = struct_value () [concrete]
 // CHECK:STDOUT:   %.115: ref %C = temporary invalid, %C.val [concrete]
 // CHECK:STDOUT:   %tuple.d3e: %tuple.type.748 = tuple_value (%C.val, %C.val) [concrete]
@@ -265,7 +262,7 @@ fn F(x: C = {});
 // CHECK:STDOUT:     %.loc6_37: %pattern_type.8b4 = tuple_pattern (%d.patt, %e.patt) [concrete = constants.%.fe8]
 // CHECK:STDOUT:     %.loc6_38: %pattern_type.4fa = tuple_pattern (%c.patt, %.loc6_37) [concrete = constants.%.972]
 // CHECK:STDOUT:     %.loc6_39: %pattern_type.f5f = tuple_pattern (%b.patt, %.loc6_38) [concrete = constants.%.341]
-// CHECK:STDOUT:     %.loc6_41: %pattern_type.f5f = default_value_pattern %.loc6_39, index: 0 [concrete = constants.%.b6d]
+// CHECK:STDOUT:     %.loc6_41: %pattern_type.f5f = default_value_pattern %.loc6_39, %.loc6_62.6 [concrete = constants.%.391]
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %.loc6_45.1: %empty_struct_type = struct_literal () [concrete = constants.%empty_struct]
 // CHECK:STDOUT:     %.loc6_50.1: %empty_struct_type = struct_literal () [concrete = constants.%empty_struct]
@@ -321,10 +318,7 @@ fn F(x: C = {});
 // CHECK:STDOUT:   }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @F(%a.param: %C, %b.param: %C, %c.param: %C, %d.param: %C, %e.param: %C) {
-// CHECK:STDOUT: !default_values:
-// CHECK:STDOUT:   %.loc6_62.6: %tuple.type.a40 = converted %.loc6_62.1, %tuple.loc6_62 [concrete = constants.%tuple.800]
-// CHECK:STDOUT: }
+// CHECK:STDOUT: fn @F(%a.param: %C, %b.param: %C, %c.param: %C, %d.param: %C, %e.param: %C);
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- basic.carbon
 // CHECK:STDOUT:
@@ -335,7 +329,7 @@ fn F(x: C = {});
 // CHECK:STDOUT:   %x.param_patt: %pattern_type = value_param_pattern [concrete]
 // CHECK:STDOUT:   %x.patt: %pattern_type = wrapper_binding_pattern x, %x.param_patt [concrete]
 // CHECK:STDOUT:   %empty_struct: %empty_struct_type = struct_value () [concrete]
-// CHECK:STDOUT:   %.015: %pattern_type = default_value_pattern %x.patt, index: 0 [concrete]
+// CHECK:STDOUT:   %.ff4: %pattern_type = default_value_pattern %x.patt, @F.%.loc6_14.6 [concrete]
 // CHECK:STDOUT:   %C.val: %C = struct_value () [concrete]
 // CHECK:STDOUT:   %.115: ref %C = temporary invalid, %C.val [concrete]
 // CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
@@ -346,7 +340,7 @@ fn F(x: C = {});
 // CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
 // CHECK:STDOUT:     %x.param_patt: %pattern_type = value_param_pattern [concrete = constants.%x.param_patt]
 // CHECK:STDOUT:     %x.patt: %pattern_type = wrapper_binding_pattern x, %x.param_patt [concrete = constants.%x.patt]
-// CHECK:STDOUT:     %.loc6_11: %pattern_type = default_value_pattern %x.patt, index: 0 [concrete = constants.%.015]
+// CHECK:STDOUT:     %.loc6_11: %pattern_type = default_value_pattern %x.patt, %.loc6_14.6 [concrete = constants.%.ff4]
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %.loc6_14.1: %empty_struct_type = struct_literal () [concrete = constants.%empty_struct]
 // CHECK:STDOUT:     %x.param: %C = value_param call_param0
@@ -360,8 +354,5 @@ fn F(x: C = {});
 // CHECK:STDOUT:   }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @F(%x.param: %C) {
-// CHECK:STDOUT: !default_values:
-// CHECK:STDOUT:   %.loc6_14.6: %C = acquire_value %.loc6_14.5 [concrete = constants.%C.val]
-// CHECK:STDOUT: }
+// CHECK:STDOUT: fn @F(%x.param: %C);
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/function/definition/default_values.carbon b/toolchain/check/testdata/function/definition/default_values.carbon
index 9218ece32e2b..8b965beef420 100644
--- a/toolchain/check/testdata/function/definition/default_values.carbon
+++ b/toolchain/check/testdata/function/definition/default_values.carbon
@@ -14,12 +14,12 @@
 library "[[@TEST_NAME]]";
 
 fn F(x: i32 = 0, y: i32 = 10);
-// CHECK:STDERR: fail_defaults_different_values_redecl.carbon:[[@LINE+7]]:41: error: redeclaration syntax differs here [RedeclParamSyntaxDiffers]
+// CHECK:STDERR: fail_defaults_different_values_redecl.carbon:[[@LINE+7]]:25: error: redeclaration differs at parameter 2 [RedeclParamDiffers]
 // CHECK:STDERR: fn F(unused x: i32 = 0, unused y: i32 = 22) { }
-// CHECK:STDERR:                                         ^~
-// CHECK:STDERR: fail_defaults_different_values_redecl.carbon:[[@LINE-4]]:27: note: comparing with previous declaration here [RedeclParamSyntaxPrevious]
+// CHECK:STDERR:                         ^~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fail_defaults_different_values_redecl.carbon:[[@LINE-4]]:18: note: previous declaration's corresponding parameter here [RedeclParamPrevious]
 // CHECK:STDERR: fn F(x: i32 = 0, y: i32 = 10);
-// CHECK:STDERR:                           ^~
+// CHECK:STDERR:                  ^~~~~~~~~~~
 // CHECK:STDERR:
 fn F(unused x: i32 = 0, unused y: i32 = 22) { }
 
@@ -49,13 +49,13 @@ fn F(x: i32 = 0, y: i32 = 10);
 // --- fail_imported_defaults_different_values.impl.carbon
 impl library "[[@TEST_NAME]]";
 
-// CHECK:STDERR: fail_imported_defaults_different_values.impl.carbon:[[@LINE+8]]:41: error: default value of `22` differs from the previously declared default value of `10` [PatternDefaultValueDiffers]
+// CHECK:STDERR: fail_imported_defaults_different_values.impl.carbon:[[@LINE+8]]:25: error: redeclaration differs at parameter 2 [RedeclParamDiffers]
 // CHECK:STDERR: fn F(unused x: i32 = 0, unused y: i32 = 22) { }
-// CHECK:STDERR:                                         ^~
+// CHECK:STDERR:                         ^~~~~~~~~~~~~~~~~~
 // CHECK:STDERR: fail_imported_defaults_different_values.impl.carbon:[[@LINE-5]]:1: in import [InImport]
-// CHECK:STDERR: imported_defaults_different_values.carbon:3:27: note: different previous declaration here [PatternDefaultValueDiffersNote]
+// CHECK:STDERR: imported_defaults_different_values.carbon:3:18: note: previous declaration's corresponding parameter here [RedeclParamPrevious]
 // CHECK:STDERR: fn F(x: i32 = 0, y: i32 = 10);
-// CHECK:STDERR:                           ^~
+// CHECK:STDERR:                  ^~~~~~~~~~~
 // CHECK:STDERR:
 fn F(unused x: i32 = 0, unused y: i32 = 22) { }
 
@@ -104,7 +104,6 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %x.param_patt: %pattern_type.6b6 = value_param_pattern [concrete]
 // CHECK:STDOUT:   %x.patt: %pattern_type.6b6 = wrapper_binding_pattern x, %x.param_patt [concrete]
 // CHECK:STDOUT:   %int_0.5c6: Core.IntLiteral = int_value 0 [concrete]
-// CHECK:STDOUT:   %.e53: %pattern_type.6b6 = default_value_pattern %x.patt, index: 0 [concrete]
 // CHECK:STDOUT:   %.795f: Core.Form = init_form %i32 [concrete]
 // CHECK:STDOUT:   %return.param_patt.a9a: %pattern_type.6b6 = out_param_pattern [concrete]
 // CHECK:STDOUT:   %return.patt.e1b: %pattern_type.6b6 = return_slot_pattern %return.param_patt.a9a, %i32 [concrete]
@@ -126,6 +125,7 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
 // CHECK:STDOUT:   %G.type: type = fn_type @G [concrete]
 // CHECK:STDOUT:   %G: %G.type = struct_value () [concrete]
+// CHECK:STDOUT:   %.d3f: %pattern_type.6b6 = default_value_pattern %x.patt, @F.%.loc7_15.2 [concrete]
 // CHECK:STDOUT:   %Copy.type: type = facet_type <@Copy> [concrete]
 // CHECK:STDOUT:   %Int.as.Copy.impl.Op.type.ac8: type = fn_type @Int.as.Copy.impl.Op, @Int.as.Copy.impl(%N) [symbolic]
 // CHECK:STDOUT:   %Int.as.Copy.impl.Op.5e0: %Int.as.Copy.impl.Op.type.ac8 = struct_value () [symbolic]
@@ -137,6 +137,7 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %.737: type = fn_type_with_self_type %Copy.WithSelf.Op.type.381, %Copy.facet [concrete]
 // CHECK:STDOUT:   %Int.as.Copy.impl.Op.specific_fn: <specific function> = specific_function %Int.as.Copy.impl.Op.4f6, @Int.as.Copy.impl.Op(%int_32) [concrete]
 // CHECK:STDOUT:   %.d49: <unspecified_value> = unspecified_value [concrete]
+// CHECK:STDOUT:   %.5aa: %pattern_type.6b6 = default_value_pattern %x.patt, <unspecified> [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -150,7 +151,7 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %F.decl.loc7: %F.type = fn_decl @F [concrete = constants.%F] {
 // CHECK:STDOUT:     %x.param_patt: %pattern_type.6b6 = value_param_pattern [concrete = constants.%x.param_patt]
 // CHECK:STDOUT:     %x.patt: %pattern_type.6b6 = wrapper_binding_pattern x, %x.param_patt [concrete = constants.%x.patt]
-// CHECK:STDOUT:     %.loc7_13: %pattern_type.6b6 = default_value_pattern %x.patt, index: 0 [concrete = constants.%.e53]
+// CHECK:STDOUT:     %.loc7_13: %pattern_type.6b6 = default_value_pattern %x.patt, %.loc7_15.2 [concrete = constants.%.d3f]
 // CHECK:STDOUT:     %return.param_patt: %pattern_type.6b6 = out_param_pattern [concrete = constants.%return.param_patt.a9a]
 // CHECK:STDOUT:     %return.patt: %pattern_type.6b6 = return_slot_pattern %return.param_patt, %i32.loc7_21 [concrete = constants.%return.patt.e1b]
 // CHECK:STDOUT:   } {
@@ -173,7 +174,7 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %G.decl.loc8: %G.type = fn_decl @G [concrete = constants.%G] {
 // CHECK:STDOUT:     %x.param_patt: %pattern_type.6b6 = value_param_pattern [concrete = constants.%x.param_patt]
 // CHECK:STDOUT:     %x.patt: %pattern_type.6b6 = wrapper_binding_pattern x, %x.param_patt [concrete = constants.%x.patt]
-// CHECK:STDOUT:     %.loc8_13: %pattern_type.6b6 = default_value_pattern %x.patt, index: 0 [concrete = constants.%.e53]
+// CHECK:STDOUT:     %.loc8_13: %pattern_type.6b6 = default_value_pattern %x.patt, <unspecified> [concrete = constants.%.5aa]
 // CHECK:STDOUT:     %return.param_patt: %pattern_type.6b6 = out_param_pattern [concrete = constants.%return.param_patt.a9a]
 // CHECK:STDOUT:     %return.patt: %pattern_type.6b6 = return_slot_pattern %return.param_patt, %i32.loc8_21 [concrete = constants.%return.patt.e1b]
 // CHECK:STDOUT:   } {
@@ -189,9 +190,6 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F(%x.param.loc7: %i32) -> out %return.param.loc7: %i32 {
-// CHECK:STDOUT: !default_values:
-// CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %x.ref: %i32 = name_ref x, %x.loc7
 // CHECK:STDOUT:   %impl.elem0.loc7_34: %.737 = impl_witness_access constants.%Copy.impl_witness.b51, element0 [concrete = constants.%Int.as.Copy.impl.Op.4f6]
@@ -203,9 +201,6 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @G(%x.param.loc8: %i32) -> out %return.param.loc8: %i32 {
-// CHECK:STDOUT: !default_values:
-// CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %x.ref: %i32 = name_ref x, %x.loc8
 // CHECK:STDOUT:   %impl.elem0.loc8: %.737 = impl_witness_access constants.%Copy.impl_witness.b51, element0 [concrete = constants.%Int.as.Copy.impl.Op.4f6]
@@ -229,7 +224,7 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %x.param_patt: %pattern_type.6b6 = value_param_pattern [concrete]
 // CHECK:STDOUT:   %x.patt: %pattern_type.6b6 = wrapper_binding_pattern x, %x.param_patt [concrete]
 // CHECK:STDOUT:   %int_0.5c6: Core.IntLiteral = int_value 0 [concrete]
-// CHECK:STDOUT:   %.e53: %pattern_type.6b6 = default_value_pattern %x.patt, index: 0 [concrete]
+// CHECK:STDOUT:   %.3d1: %pattern_type.6b6 = default_value_pattern %x.patt, @F.%.loc4_15.2 [concrete]
 // CHECK:STDOUT:   %.795f: Core.Form = init_form %i32 [concrete]
 // CHECK:STDOUT:   %return.param_patt.a9a: %pattern_type.6b6 = out_param_pattern [concrete]
 // CHECK:STDOUT:   %return.patt.e1b: %pattern_type.6b6 = return_slot_pattern %return.param_patt.a9a, %i32 [concrete]
@@ -269,7 +264,7 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
 // CHECK:STDOUT:     %x.param_patt: %pattern_type.6b6 = value_param_pattern [concrete = constants.%x.param_patt]
 // CHECK:STDOUT:     %x.patt: %pattern_type.6b6 = wrapper_binding_pattern x, %x.param_patt [concrete = constants.%x.patt]
-// CHECK:STDOUT:     %.loc4_13: %pattern_type.6b6 = default_value_pattern %x.patt, index: 0 [concrete = constants.%.e53]
+// CHECK:STDOUT:     %.loc4_13: %pattern_type.6b6 = default_value_pattern %x.patt, %.loc4_15.2 [concrete = constants.%.3d1]
 // CHECK:STDOUT:     %return.param_patt: %pattern_type.6b6 = out_param_pattern [concrete = constants.%return.param_patt.a9a]
 // CHECK:STDOUT:     %return.patt: %pattern_type.6b6 = return_slot_pattern %return.param_patt, %i32.loc4_21 [concrete = constants.%return.patt.e1b]
 // CHECK:STDOUT:   } {
@@ -292,9 +287,6 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F(%x.param: %i32) -> out %return.param: %i32 [from "imported_repeated_defaults.carbon"] {
-// CHECK:STDOUT: !default_values:
-// CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %x.ref: %i32 = name_ref x, %x
 // CHECK:STDOUT:   %impl.elem0.loc4_34: %.737 = impl_witness_access constants.%Copy.impl_witness.b51, element0 [concrete = constants.%Int.as.Copy.impl.Op.4f6]
@@ -315,7 +307,7 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %x.param_patt: %pattern_type.6b6 = value_param_pattern [concrete]
 // CHECK:STDOUT:   %x.patt: %pattern_type.6b6 = wrapper_binding_pattern x, %x.param_patt [concrete]
 // CHECK:STDOUT:   %.d49: <unspecified_value> = unspecified_value [concrete]
-// CHECK:STDOUT:   %.e53: %pattern_type.6b6 = default_value_pattern %x.patt, index: 0 [concrete]
+// CHECK:STDOUT:   %.c5e: %pattern_type.6b6 = default_value_pattern %x.patt, <unspecified> [concrete]
 // CHECK:STDOUT:   %.795f: Core.Form = init_form %i32 [concrete]
 // CHECK:STDOUT:   %return.param_patt.a9a: %pattern_type.6b6 = out_param_pattern [concrete]
 // CHECK:STDOUT:   %return.patt.e1b: %pattern_type.6b6 = return_slot_pattern %return.param_patt.a9a, %i32 [concrete]
@@ -342,7 +334,7 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
 // CHECK:STDOUT:     %x.param_patt: %pattern_type.6b6 = value_param_pattern [concrete = constants.%x.param_patt]
 // CHECK:STDOUT:     %x.patt: %pattern_type.6b6 = wrapper_binding_pattern x, %x.param_patt [concrete = constants.%x.patt]
-// CHECK:STDOUT:     %.loc4_13: %pattern_type.6b6 = default_value_pattern %x.patt, index: 0 [concrete = constants.%.e53]
+// CHECK:STDOUT:     %.loc4_13: %pattern_type.6b6 = default_value_pattern %x.patt, <unspecified> [concrete = constants.%.c5e]
 // CHECK:STDOUT:     %return.param_patt: %pattern_type.6b6 = out_param_pattern [concrete = constants.%return.param_patt.a9a]
 // CHECK:STDOUT:     %return.patt: %pattern_type.6b6 = return_slot_pattern %return.param_patt, %i32.loc4_21 [concrete = constants.%return.patt.e1b]
 // CHECK:STDOUT:   } {
@@ -358,9 +350,6 @@ fn F(x: i32 = _) -> i32 { return x; }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F(%x.param: %i32) -> out %return.param: %i32 [from "imported_elided_defaults_in_def.carbon"] {
-// CHECK:STDOUT: !default_values:
-// CHECK:STDOUT:   <elided>
-// CHECK:STDOUT:
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %x.ref: %i32 = name_ref x, %x
 // CHECK:STDOUT:   %impl.elem0: %.737 = impl_witness_access constants.%Copy.impl_witness.b51, element0 [concrete = constants.%Int.as.Copy.impl.Op.4f6]
diff --git a/toolchain/check/thunk.cpp b/toolchain/check/thunk.cpp
index 61cbae832e94..e2577575d0ab 100644
--- a/toolchain/check/thunk.cpp
+++ b/toolchain/check/thunk.cpp
@@ -249,7 +249,6 @@ static auto CloneFunctionDecl(Context& context, SemIR::LocId loc_id,
           {
               .call_param_patterns_id = match_results.call_param_patterns_id,
               .call_params_id = match_results.call_params_id,
-              .call_param_default_values_id = SemIR::InstBlockId::Empty,
               .call_param_ranges = match_results.param_ranges,
               .return_type_inst_id = return_type_inst_id,
               .return_form_inst_id = return_form_inst_id,
diff --git a/toolchain/diagnostics/kind.def b/toolchain/diagnostics/kind.def
index e9ec0b695e3a..3e0e28f845ab 100644
--- a/toolchain/diagnostics/kind.def
+++ b/toolchain/diagnostics/kind.def
@@ -620,8 +620,6 @@ CARBON_DIAGNOSTIC_KIND(GenericMissingExplicitParameters)
 CARBON_DIAGNOSTIC_KIND(RequiredPatternDefaultValueMissing)
 CARBON_DIAGNOSTIC_KIND(RequiredPatternDefaultValueFirstDefault)
 CARBON_DIAGNOSTIC_KIND(RequiredPatternDefaultValueMissingAdditional)
-CARBON_DIAGNOSTIC_KIND(PatternDefaultValueDiffers)
-CARBON_DIAGNOSTIC_KIND(PatternDefaultValueDiffersNote)
 CARBON_DIAGNOSTIC_KIND(PatternDefaultValueNotConstant)
 CARBON_DIAGNOSTIC_KIND(PatternDefaultValueNotInParameterList)
 CARBON_DIAGNOSTIC_KIND(PatternDefaultValueNotSpecified)
diff --git a/toolchain/driver/testdata/stdin.carbon b/toolchain/driver/testdata/stdin.carbon
index e5857613b394..c8f4c5e83a01 100644
--- a/toolchain/driver/testdata/stdin.carbon
+++ b/toolchain/driver/testdata/stdin.carbon
@@ -33,6 +33,7 @@
 // CHECK:STDOUT:   import_ir_insts: {}
 // CHECK:STDOUT:   clang_decls:     {}
 // CHECK:STDOUT:   clang_decl_signatures: {}
+// CHECK:STDOUT:   default_values:  {}
 // CHECK:STDOUT:   name_scopes:
 // CHECK:STDOUT:     name_scope0:     {inst: inst10, parent_scope: name_scope<none>, has_error: false, extended_scopes: [], names: {}}
 // CHECK:STDOUT:   entity_names:    {}
diff --git a/toolchain/sem_ir/BUILD b/toolchain/sem_ir/BUILD
index c08c174cf2dc..83bc914226f1 100644
--- a/toolchain/sem_ir/BUILD
+++ b/toolchain/sem_ir/BUILD
@@ -100,6 +100,7 @@ cc_library(
         "cpp_initializer_list.cpp",
         "cpp_overload_set.cpp",
         "declared_facet_type.cpp",
+        "default_value.cpp",
         "entity_name.cpp",
         "field.cpp",
         "file.cpp",
@@ -135,6 +136,7 @@ cc_library(
         "cpp_initializer_list.h",
         "cpp_overload_set.h",
         "declared_facet_type.h",
+        "default_value.h",
         "entity_name.h",
         "entity_with_params_base.h",
         "field.h",
diff --git a/toolchain/sem_ir/default_value.cpp b/toolchain/sem_ir/default_value.cpp
new file mode 100644
index 000000000000..7e968c6fdc3c
--- /dev/null
+++ b/toolchain/sem_ir/default_value.cpp
@@ -0,0 +1,12 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/sem_ir/default_value.h"
+
+#include "toolchain/base/value_store_impl.h"
+
+namespace Carbon {
+template class ValueStore<SemIR::DefaultValueId, SemIR::DefaultValue,
+                          Tag<SemIR::CheckIRId>>;
+}  // namespace Carbon
diff --git a/toolchain/sem_ir/default_value.h b/toolchain/sem_ir/default_value.h
new file mode 100644
index 000000000000..db2a67bf402c
--- /dev/null
+++ b/toolchain/sem_ir/default_value.h
@@ -0,0 +1,45 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef CARBON_TOOLCHAIN_SEM_IR_DEFAULT_VALUE_H_
+#define CARBON_TOOLCHAIN_SEM_IR_DEFAULT_VALUE_H_
+
+#include "toolchain/base/value_store.h"
+#include "toolchain/sem_ir/ids.h"
+
+namespace Carbon::SemIR {
+
+// Information about a default value for a pattern, such as default values for
+// function parameters.
+struct DefaultValue : public Printable<DefaultValue> {
+  auto Print(llvm::raw_ostream& out) const -> void {
+    out << "{raw_id: " << raw_id << ", value_id: " << value_id
+        << ", is_unspecified: " << is_unspecified << "}";
+  }
+
+  // The instruction specifying the default value as specified by the developer,
+  // before conversion to the pattern scrutinee type is applied.
+  InstId raw_id;
+
+  // The instruction specifying the default value in the same type as the
+  // scrutinee type.
+  InstId value_id;
+
+  // Whether the user left this default value unspecified. The `value_id` will
+  // be `None` but the `raw_id` will contain the `UnspecifiedValue` instruction
+  // for use in diagnostics if needed.
+  bool is_unspecified;
+};
+
+using DefaultValueStore =
+    ValueStore<DefaultValueId, DefaultValue, Tag<SemIR::CheckIRId>>;
+
+}  // namespace Carbon::SemIR
+
+namespace Carbon {
+extern template class ValueStore<SemIR::DefaultValueId, SemIR::DefaultValue,
+                                 Tag<SemIR::CheckIRId>>;
+}
+
+#endif  // CARBON_TOOLCHAIN_SEM_IR_DEFAULT_VALUE_H_
diff --git a/toolchain/sem_ir/dump.cpp b/toolchain/sem_ir/dump.cpp
index 17fb69847f6f..6cc6443c55f3 100644
--- a/toolchain/sem_ir/dump.cpp
+++ b/toolchain/sem_ir/dump.cpp
@@ -255,17 +255,6 @@ LLVM_DUMP_METHOD auto Dump(const File& file, CppOverloadSetId overload_set_id)
   return out.TakeStr();
 }
 
-LLVM_DUMP_METHOD auto Dump(const File& file, EntityNameId entity_name_id)
-    -> std::string {
-  RawStringOstream out;
-  out << entity_name_id;
-  if (entity_name_id.has_value()) {
-    auto entity_name = file.entity_names().Get(entity_name_id);
-    out << ": " << entity_name << DumpNameOfEntityName(file, entity_name);
-  }
-  return out.TakeStr();
-}
-
 LLVM_DUMP_METHOD auto Dump(const File& file,
                            DeclaredFacetTypeId declared_facet_type_id)
     -> std::string {
@@ -314,6 +303,30 @@ LLVM_DUMP_METHOD auto Dump(const File& file,
   return out.TakeStr();
 }
 
+LLVM_DUMP_METHOD auto Dump(const File& file, DefaultValueId value_id)
+    -> std::string {
+  RawStringOstream out;
+  out << value_id;
+  if (value_id.has_value()) {
+    const auto& default_value = file.default_values().Get(value_id);
+    out << ": " << default_value
+        << "\n  - raw: " << DumpInstSummary(file, default_value.raw_id)
+        << "\n  - value: " << DumpInstSummary(file, default_value.value_id);
+  }
+  return out.TakeStr();
+}
+
+LLVM_DUMP_METHOD auto Dump(const File& file, EntityNameId entity_name_id)
+    -> std::string {
+  RawStringOstream out;
+  out << entity_name_id;
+  if (entity_name_id.has_value()) {
+    auto entity_name = file.entity_names().Get(entity_name_id);
+    out << ": " << entity_name << DumpNameOfEntityName(file, entity_name);
+  }
+  return out.TakeStr();
+}
+
 LLVM_DUMP_METHOD auto Dump(const File& file, FieldId field_id) -> std::string {
   RawStringOstream out;
   out << field_id;
@@ -655,6 +668,9 @@ LLVM_DUMP_METHOD static auto MakeConstantId(int id) -> ConstantId {
 LLVM_DUMP_METHOD auto MakeSymbolicConstantId(int id) -> ConstantId {
   return ConstantId::ForSymbolicConstantId(ConstantId::SymbolicId(id));
 }
+LLVM_DUMP_METHOD static auto MakeDefaultValueId(int id) -> DefaultValueId {
+  return DefaultValueId(id);
+}
 LLVM_DUMP_METHOD static auto MakeEntityNameId(int id) -> EntityNameId {
   return EntityNameId(id);
 }
diff --git a/toolchain/sem_ir/dump.h b/toolchain/sem_ir/dump.h
index 818b453355bd..36acbb5c77cc 100644
--- a/toolchain/sem_ir/dump.h
+++ b/toolchain/sem_ir/dump.h
@@ -28,9 +28,10 @@ auto Dump(const File& file, GeneratedFunctionId generated_function_id)
 auto Dump(const File& file, ClassId class_id) -> std::string;
 auto Dump(const File& file, ConstantId const_id) -> std::string;
 auto Dump(const File& file, CppOverloadSetId overload_set_id) -> std::string;
-auto Dump(const File& file, EntityNameId entity_name_id) -> std::string;
 auto Dump(const File& file, DeclaredFacetTypeId declared_facet_type_id)
     -> std::string;
+auto Dump(const File& file, DefaultValueId default_value_id) -> std::string;
+auto Dump(const File& file, EntityNameId entity_name_id) -> std::string;
 auto Dump(const File& file, FieldId field_id) -> std::string;
 auto Dump(const File& file, FunctionId function_id) -> std::string;
 auto Dump(const File& file, GenericId generic_id) -> std::string;
diff --git a/toolchain/sem_ir/file.cpp b/toolchain/sem_ir/file.cpp
index 0b6fa2cb63a8..041c7efc3ead 100644
--- a/toolchain/sem_ir/file.cpp
+++ b/toolchain/sem_ir/file.cpp
@@ -40,6 +40,7 @@ File::File(const Parse::Tree* parse_tree, CheckIRId check_ir_id,
       value_stores_(&value_stores),
       filename_(std::move(filename)),
       entity_names_(check_ir_id),
+      default_values_(check_ir_id),
       functions_(check_ir_id),
       generated_functions_(check_ir_id),
       cpp_overload_sets_(check_ir_id),
@@ -160,6 +161,7 @@ auto File::OutputYaml(bool include_singletons) const -> Yaml::OutputMapping {
           map.Add("import_ir_insts", import_ir_insts_.OutputYaml());
           map.Add("clang_decls", clang_decls_.OutputYaml());
           map.Add("clang_decl_signatures", clang_decl_signatures_.OutputYaml());
+          map.Add("default_values", default_values_.OutputYaml());
           map.Add("name_scopes", name_scopes_.OutputYaml());
           map.Add("entity_names", entity_names_.OutputYaml());
           map.Add("functions", functions_.OutputYaml());
@@ -200,6 +202,8 @@ auto File::CollectMemUsage(MemUsage& mem_usage, llvm::StringRef label) const
   mem_usage.Collect(MemUsage::ConcatLabel(label, "functions_"), functions_);
   mem_usage.Collect(MemUsage::ConcatLabel(label, "thunks_"), thunks_);
   mem_usage.Collect(MemUsage::ConcatLabel(label, "classes_"), classes_);
+  mem_usage.Collect(MemUsage::ConcatLabel(label, "default_values_"),
+                    default_values_);
   mem_usage.Collect(MemUsage::ConcatLabel(label, "interfaces_"), interfaces_);
   mem_usage.Collect(MemUsage::ConcatLabel(label, "impls_"), impls_);
   mem_usage.Collect(MemUsage::ConcatLabel(label, "generics_"), generics_);
diff --git a/toolchain/sem_ir/file.h b/toolchain/sem_ir/file.h
index 5219747dc481..8328bb0b4dc9 100644
--- a/toolchain/sem_ir/file.h
+++ b/toolchain/sem_ir/file.h
@@ -25,6 +25,7 @@
 #include "toolchain/sem_ir/cpp_file.h"
 #include "toolchain/sem_ir/cpp_overload_set.h"
 #include "toolchain/sem_ir/declared_facet_type.h"
+#include "toolchain/sem_ir/default_value.h"
 #include "toolchain/sem_ir/entity_name.h"
 #include "toolchain/sem_ir/field.h"
 #include "toolchain/sem_ir/function.h"
@@ -222,6 +223,10 @@ class File : public Printable<File> {
   auto declared_facet_types() const -> const DeclaredFacetTypeStore& {
     return declared_facet_types_;
   }
+  auto default_values() const -> const DefaultValueStore& {
+    return default_values_;
+  }
+  auto default_values() -> DefaultValueStore& { return default_values_; }
 
   // If `class_id` is an imported C++ class, appends the Clang mangled name of
   // its type to `out` and returns true. Otherwise returns false and leaves
@@ -365,6 +370,9 @@ class File : public Printable<File> {
   // Storage for EntityNames.
   EntityNameStore entity_names_;
 
+  // Storage for DefaultValues.
+  DefaultValueStore default_values_;
+
   // Storage for callable objects.
   FunctionStore functions_;
 
diff --git a/toolchain/sem_ir/formatter.cpp b/toolchain/sem_ir/formatter.cpp
index 65dc93792e56..670d891495d0 100644
--- a/toolchain/sem_ir/formatter.cpp
+++ b/toolchain/sem_ir/formatter.cpp
@@ -598,17 +598,10 @@ auto Formatter::FormatFunction(FunctionId id, const Function& fn) -> void {
     out() << "]";
   }
 
-  if (!fn.body_block_ids.empty() ||
-      fn.call_param_default_values_id != SemIR::InstBlockId::Empty) {
+  if (!fn.body_block_ids.empty()) {
     out() << ' ';
     OpenBrace();
 
-    if (fn.call_param_default_values_id != SemIR::InstBlockId::Empty) {
-      IndentLabel();
-      out() << "!default_values:\n";
-      FormatCodeBlock(fn.call_param_default_values_id);
-    }
-
     for (auto block_id : fn.body_block_ids) {
       IndentLabel();
       FormatLabel(block_id);
@@ -1646,7 +1639,12 @@ auto Formatter::FormatArg(DeclaredFacetTypeId id) -> void {
 }
 
 auto Formatter::FormatArg(DefaultValueId id) -> void {
-  out() << "index: " << id.index;
+  const auto& default_value = sem_ir_->default_values().Get(id);
+  if (default_value.is_unspecified) {
+    out() << "<unspecified>";
+  } else {
+    FormatArg(default_value.value_id);
+  }
 }
 
 auto Formatter::FormatArg(FieldId id) -> void {
diff --git a/toolchain/sem_ir/function.h b/toolchain/sem_ir/function.h
index 18338ef0fe78..52ec547d89b3 100644
--- a/toolchain/sem_ir/function.h
+++ b/toolchain/sem_ir/function.h
@@ -82,9 +82,6 @@ struct FunctionFields {
   // because it is relevant only for a function definition.
   InstBlockId call_params_id;
 
-  // Instructions representing the default values for parameters.
-  InstBlockId call_param_default_values_id;
-
   // The index ranges within the `Call` parameters that correspond to the
   // implicit parameters, explicit parameters, and return.
   //
@@ -234,9 +231,6 @@ struct Function : public EntityWithParamsBase,
     if (call_params_id.has_value()) {
       out << ", call_params_id: " << call_params_id;
     }
-    if (call_param_default_values_id != SemIR::InstBlockId::Empty) {
-      out << ", call_param_default_values_id: " << call_param_default_values_id;
-    }
     if (return_type_inst_id.has_value()) {
       out << ", return_type_inst_id: " << return_type_inst_id;
     }
diff --git a/toolchain/sem_ir/ids.h b/toolchain/sem_ir/ids.h
index 93d5c0e4ad28..ba0539a3f63c 100644
--- a/toolchain/sem_ir/ids.h
+++ b/toolchain/sem_ir/ids.h
@@ -116,11 +116,11 @@ class AbsoluteInstId : public InstId {
   using InstId::InstId;
 };
 
-// An id representing the index of the default value constant instruction in
-// a default values instruction block.
+// The ID of a default value.
 class DefaultValueId : public IdBase<DefaultValueId> {
  public:
-  static constexpr llvm::StringLiteral Label = "default_value_id";
+  static constexpr llvm::StringLiteral Label = "default_value";
+
   using IdBase::IdBase;
 };
 
diff --git a/toolchain/sem_ir/yaml_test.cpp b/toolchain/sem_ir/yaml_test.cpp
index 38da393cd779..f5e0b447e75f 100644
--- a/toolchain/sem_ir/yaml_test.cpp
+++ b/toolchain/sem_ir/yaml_test.cpp
@@ -62,6 +62,7 @@ TEST(SemIRTest, Yaml) {
       Pair("import_ir_insts", Yaml::Mapping(SizeIs(0))),
       Pair("clang_decls", Yaml::Mapping(SizeIs(0))),
       Pair("clang_decl_signatures", Yaml::Mapping(SizeIs(0))),
+      Pair("default_values", Yaml::Mapping(SizeIs(0))),
       Pair("name_scopes", Yaml::Mapping(SizeIs(1))),
       Pair("entity_names", Yaml::Mapping(SizeIs(1))),
       Pair("functions", Yaml::Mapping(SizeIs(1))),