diff --git a/config.h b/config.h
index 51657c05..6b607e4e 100644
--- a/config.h
+++ b/config.h
@@ -355,11 +355,15 @@ NAMESPACE_END
 #endif
 
 // Sun Studio Express 3 (December 2006) provides GCC-style attributes.
+// IBM XL C/C++ alignment modifier per Optimization Guide, pp. 19-20.
+// CRYPTOPP_ALIGN_DATA may not be reliable on AIX.
 #ifndef CRYPTOPP_ALIGN_DATA
 	#if defined(_MSC_VER)
 		#define CRYPTOPP_ALIGN_DATA(x) __declspec(align(x))
 	#elif defined(__GNUC__) || (__SUNPRO_CC >= 0x5100)
 		#define CRYPTOPP_ALIGN_DATA(x) __attribute__((aligned(x)))
+	#elif defined(__xlc__) || defined(__xlC__)
+		#define CRYPTOPP_ALIGN_DATA(x) __attribute__((aligned(x)))
 	#else
 		#define CRYPTOPP_ALIGN_DATA(x)
 	#endif