diff --git a/reactos/tools/rbuild/backend/mingw/modulehandler.cpp b/reactos/tools/rbuild/backend/mingw/modulehandler.cpp
index 96bfc2cc19c..e133918a042 100644
--- a/reactos/tools/rbuild/backend/mingw/modulehandler.cpp
+++ b/reactos/tools/rbuild/backend/mingw/modulehandler.cpp
@@ -813,25 +813,35 @@ MingwModuleHandler::GenerateMacro (
 	}
 	for ( i = 0; i < data.defines.size(); i++ )
 	{
-		const Define& d = *data.defines[i];
-		if (used_defs && used_defs->find(d.name) != used_defs->end())
+		const Define& define = *data.defines[i];
+		if ( used_defs && used_defs->find ( define.name ) != used_defs->end () )
 		{
+#if 0 /* FIXME: activate */
+			if ( !define.overridable )
+			{
+				throw InvalidOperationException ( __FILE__,
+				                                  __LINE__,
+				                                  "Invalid override of define '%s' in '%s'",
+				                                  define.name.c_str (),
+				                                  module.name.c_str () );
+			}
+#endif
 			if ( backend->configuration.Verbose )
 				printf("%s define overridden in '%s' module\n",
-					d.name.c_str (), module.name.c_str () );
+					define.name.c_str (), module.name.c_str () );
 			continue;
 		}
 		fprintf (
 			fMakefile,
 			" -D%s",
-			d.name.c_str() );
-		if ( d.value.size() )
+			define.name.c_str() );
+		if ( define.value.size() )
 			fprintf (
 				fMakefile,
 				"=%s",
-				d.value.c_str() );
-		if (used_defs)
-			used_defs->insert(used_defs->begin(), d.name);
+				define.value.c_str() );
+		if ( used_defs )
+			used_defs->insert(used_defs->begin(), define.name);
 	}
 	if ( generateAssignment )
 	{
diff --git a/reactos/tools/rbuild/define.cpp b/reactos/tools/rbuild/define.cpp
index 03ec41187c3..d071a749ca7 100644
--- a/reactos/tools/rbuild/define.cpp
+++ b/reactos/tools/rbuild/define.cpp
@@ -63,12 +63,21 @@ void
 Define::Initialize()
 {
 	const XMLAttribute* att = node->GetAttribute ( "name", true );
-	const XMLAttribute* bck = node->GetAttribute ( "backend", false );
+
+	att = node->GetAttribute ( "name", true );
 	assert(att);
 	name = att->value;
 	value = node->value;
-	if ( bck )
-		backend = bck->value;
+
+	att = node->GetAttribute ( "backend", false );
+	if ( att )
+		backend = att->value;
+
+	att = node->GetAttribute ( "overridable", false );
+	if ( att )
+		overridable = ( att->value == "true" || att->value == "yes" );
+	else
+		overridable = false;
 }
 
 void
diff --git a/reactos/tools/rbuild/project.dtd b/reactos/tools/rbuild/project.dtd
index fb757c617b2..5411ce18646 100644
--- a/reactos/tools/rbuild/project.dtd
+++ b/reactos/tools/rbuild/project.dtd
@@ -37,6 +37,7 @@
 <!ELEMENT define (#PCDATA)>
 <!ATTLIST define
 	name  %CIdentifier; #REQUIRED
+	overridable (true) #IMPLIED
 >
 
 <!ELEMENT directory (compilationunit*|directory*|group*|cdfile|file*|if*|ifnot*|module+|pch*|xi:include*)+>
diff --git a/reactos/tools/rbuild/rbuild.h b/reactos/tools/rbuild/rbuild.h
index f00745e2228..f94b82a8d9f 100644
--- a/reactos/tools/rbuild/rbuild.h
+++ b/reactos/tools/rbuild/rbuild.h
@@ -426,6 +426,7 @@ public:
 	std::string name;
 	std::string value;
 	std::string backend;
+	bool overridable;
 
 	Define ( const Project& project,
 	         const XMLElement& defineNode );