diff --git a/include/boost/config/platform/macos.hpp b/include/boost/config/platform/macos.hpp
index 424027c4..2fbea49e 100644
--- a/include/boost/config/platform/macos.hpp
+++ b/include/boost/config/platform/macos.hpp
@@ -9,8 +9,9 @@
 
 #define BOOST_PLATFORM "Mac OS"
 
-// If __MACH__, we're using the BSD standard C library, not the MSL:
-#if __MACH__
+#if __MACH__ && !defined(_MSL_USING_MSL_C)
+
+// Using the Mac OS X system BSD-style C library.
 
 #  define BOOST_NO_CTYPE_FUNCTIONS
 #  define BOOST_NO_CWCHAR
@@ -41,6 +42,8 @@
 
 #else
 
+// Using the MSL C library.
+
 // We will eventually support threads in non-Carbon builds, but we do
 // not support this yet.
 #  if TARGET_CARBON