--- a/boost/config/stdlib/libstdcpp3.hpp
+++ b/boost/config/stdlib/libstdcpp3.hpp
@@ -35,7 +35,8 @@
 #  if defined(_GLIBCXX_HAVE_GTHR_DEFAULT) \
         || defined(_GLIBCXX__PTHREADS) \
         || defined(_GLIBCXX_HAS_GTHREADS) \
-        || defined(_WIN32)
+        || defined(_WIN32) \
+       || defined(_AIX)
       //
       // If the std lib has thread support turned on, then turn it on in Boost
       // as well.  We do this because some gcc-3.4 std lib headers define _REENTANT