diff --git a/glib/glibconfig.h.win32.in b/glib/glibconfig.h.win32.in
index 9a91c5e96..a8d8c0c35 100644
--- a/glib/glibconfig.h.win32.in
+++ b/glib/glibconfig.h.win32.in
@@ -219,10 +219,19 @@ typedef unsigned __int64 guintptr;
 #define GUINT_TO_LE(val)	((guint) GUINT32_TO_LE (val))
 #define GINT_TO_BE(val)		((gint) GINT32_TO_BE (val))
 #define GUINT_TO_BE(val)	((guint) GUINT32_TO_BE (val))
-#define GSIZE_TO_LE(val)	((gsize) GUINT32_TO_LE (val))
-#define GSSIZE_TO_LE(val)	((gssize) GINT32_TO_LE (val))
-#define GSIZE_TO_BE(val)	((gsize) GUINT32_TO_BE (val))
-#define GSSIZE_TO_BE(val)	((gssize) GINT32_TO_BE (val))
+
+#ifdef _WIN64
+# define GSIZE_TO_LE(val)	((gsize) GUINT64_TO_LE (val))
+# define GSSIZE_TO_LE(val)	((gssize) GINT64_TO_LE (val))
+# define GSIZE_TO_BE(val)	((gsize) GUINT64_TO_BE (val))
+# define GSSIZE_TO_BE(val)	((gssize) GINT64_TO_BE (val))
+#else
+# define GSIZE_TO_LE(val)	((gsize) GUINT32_TO_LE (val))
+# define GSSIZE_TO_LE(val)	((gssize) GINT32_TO_LE (val))
+# define GSIZE_TO_BE(val)	((gsize) GUINT32_TO_BE (val))
+# define GSSIZE_TO_BE(val)	((gssize) GINT32_TO_BE (val))
+#endif
+
 #define G_BYTE_ORDER G_LITTLE_ENDIAN
 
 #define GLIB_SYSDEF_POLLIN =1