diff --git a/include/STTypes.h b/include/STTypes.h
index 7888c4e..3ed12d9 100644
--- a/include/STTypes.h
+++ b/include/STTypes.h
@@ -143,8 +143,10 @@ namespace soundtouch
         #endif // SOUNDTOUCH_FLOAT_SAMPLES
 
         #ifdef SOUNDTOUCH_ALLOW_X86_OPTIMIZATIONS
-            // Allow MMX optimizations
-            #define SOUNDTOUCH_ALLOW_MMX   1
+            // Allow MMX optimizations (not available in X64 mode)
+            #if (!_M_X64)
+                #define SOUNDTOUCH_ALLOW_MMX   1
+            #endif
         #endif
 
     #else