diff --git a/mps/code/config.h b/mps/code/config.h
index 2b14da1b289..060c40ad604 100644
--- a/mps/code/config.h
+++ b/mps/code/config.h
@@ -171,7 +171,7 @@
  * CONFIG_PLINTH_NONE tells mps.c to exclude the ANSI plinth and removes
  * all standard C library dependencies.  e.g.
  *
- *     cc -O3 -c -DCONFIG_PLINTH_NONE mps.c
+ *     cc -O2 -c -DCONFIG_PLINTH_NONE mps.c
  */
 
 #if defined(CONFIG_PLINTH_NONE)
diff --git a/mps/readme.txt b/mps/readme.txt
index 87e685b56a9..5c94b234786 100644
--- a/mps/readme.txt
+++ b/mps/readme.txt
@@ -48,7 +48,7 @@ The MPS Kit is distributed in source form.  You need to build it before
 using it.  The basic case is straightforward on supported platforms
 (see below).  Change to the code directory then
 
-    cc -O3 -c mps.c     Unix / Mac OS X (with Xcode command line)
+    cc -O2 -c mps.c     Unix / Mac OS X (with Xcode command line)
     cl /O2 /c mps.c     Windows (with Microsoft SDK or Visual Studio 2010)
 
 This will produce an object file you can link with your project.  For