diff --git a/configure.in b/configure.in
index 7993b7a04891..fcaf90662ddb 100644
--- a/configure.in
+++ b/configure.in
@@ -3989,6 +3989,17 @@ case "$target" in
 	    fi
 	fi
 	AC_SUBST(AIX_SHLIB_BIN)
+
+	dnl ========================================================
+	dnl Figure out if we are using a pre-5.0x version of the
+	dnl xlC compiler and if so, set flag
+	dnl ========================================================
+
+	if test ! "$GNU_CC" && test "$ac_cv_cpp_reinterpret_cast" = no ; then
+	   CXXFLAGS="$CXXFLAGS -DAIX_XLC_364"
+	   CFLAGS="$CFLAGS -DAIX_XLC_364"
+	fi
+
 	;;
 
 esac
diff --git a/layout/generic/nsLineLayout.h b/layout/generic/nsLineLayout.h
index deb45a08090b..885c10854a71 100644
--- a/layout/generic/nsLineLayout.h
+++ b/layout/generic/nsLineLayout.h
@@ -381,7 +381,7 @@ protected:
   PerFrameData* mFrameFreeList;
   PRInt32 mInitialFramesFreed;
 
-#if defined(AIX) || defined(XP_OS2_VACPP)
+#if defined(AIX_XLC_364) || defined(XP_OS2_VACPP)
 public:
 #endif
   struct PerSpanData {
@@ -419,7 +419,7 @@ public:
       mLastFrame = pfd;
     }
   };
-#if defined(AIX) || defined(XP_OS2_VACPP)
+#if defined(AIX_XLC_364) || defined(XP_OS2_VACPP)
 protected:
 #endif
   PerSpanData mSpanDataBuf[NS_LINELAYOUT_NUM_SPANS];
diff --git a/layout/html/base/src/nsLineLayout.h b/layout/html/base/src/nsLineLayout.h
index deb45a08090b..885c10854a71 100644
--- a/layout/html/base/src/nsLineLayout.h
+++ b/layout/html/base/src/nsLineLayout.h
@@ -381,7 +381,7 @@ protected:
   PerFrameData* mFrameFreeList;
   PRInt32 mInitialFramesFreed;
 
-#if defined(AIX) || defined(XP_OS2_VACPP)
+#if defined(AIX_XLC_364) || defined(XP_OS2_VACPP)
 public:
 #endif
   struct PerSpanData {
@@ -419,7 +419,7 @@ public:
       mLastFrame = pfd;
     }
   };
-#if defined(AIX) || defined(XP_OS2_VACPP)
+#if defined(AIX_XLC_364) || defined(XP_OS2_VACPP)
 protected:
 #endif
   PerSpanData mSpanDataBuf[NS_LINELAYOUT_NUM_SPANS];
diff --git a/widget/src/gtk/nsKeyboardUtils.cpp b/widget/src/gtk/nsKeyboardUtils.cpp
index 27dcd3cc6b13..56a41d19f78d 100644
--- a/widget/src/gtk/nsKeyboardUtils.cpp
+++ b/widget/src/gtk/nsKeyboardUtils.cpp
@@ -18,7 +18,7 @@
 
 #include <X11/Xlib.h>
 #ifdef HAVE_X11_XKBLIB_H
-#   if defined (__digital__) && defined (__unix__)
+#   if (defined (__digital__) && defined (__unix__)) || defined(_AIX) || defined(__hpux)
 #   define explicit Explicit
 #   include <X11/XKBlib.h>
 #   undef explicit
diff --git a/xpcom/base/nscore.h b/xpcom/base/nscore.h
index b7a52611036b..9a8e641701e4 100644
--- a/xpcom/base/nscore.h
+++ b/xpcom/base/nscore.h
@@ -205,7 +205,7 @@ typedef PRUint32 nsresult;
 #define HAVE_CPP_TYPENAME
 
   /* waiting to find out if OS/2 VisualAge participates in autoconf */
-#if defined(XP_OS2_VACPP) || defined(AIX) || (defined(IRIX) && !defined(__GNUC__))
+#if defined(XP_OS2_VACPP) || defined(AIX_XLC_364) || (defined(IRIX) && !defined(__GNUC__))
   #undef HAVE_CPP_TYPENAME
 #endif
 
diff --git a/xpcom/glue/nsCOMPtr.h b/xpcom/glue/nsCOMPtr.h
index 17d6d507a09c..b0e8bcd8f073 100644
--- a/xpcom/glue/nsCOMPtr.h
+++ b/xpcom/glue/nsCOMPtr.h
@@ -192,7 +192,7 @@ class nsDerivedSafe : public T
       NS_IMETHOD_(nsrefcnt) Release(void);
 #endif
 
-#if !defined(XP_OS2_VACPP) && !defined(AIX)
+#if !defined(XP_OS2_VACPP) && !defined(AIX_XLC_364)
       void operator delete( void*, size_t );                  // NOT TO BE IMPLEMENTED
         // declaring |operator delete| private makes calling delete on an interface pointer a compile error
 #endif