diff -r eb67c42be41d Lib/tarfile.py
--- a/Lib/tarfile.py	Fri Jun 28 11:14:13 2013 +0200
+++ b/Lib/tarfile.py	Sun Jul 28 10:37:22 2013 +0200
@@ -1497,7 +1497,8 @@
         else:
             if name is None and hasattr(fileobj, "name"):
                 name = fileobj.name
-            if hasattr(fileobj, "mode"):
+            # when fileobj is a gzip.GzipFile, fileobj.mode is an int (not valid for us)
+            if hasattr(fileobj, "mode") and isinstance(fileobj.mode, str):
                 self._mode = fileobj.mode
             self._extfileobj = True
         self.name = os.path.abspath(name) if name else None