diff --git a/Lib/gzip.py b/Lib/gzip.py
index 7f56153..74c0d26 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -61,7 +61,7 @@
                                              zlib.DEF_MEM_LEVEL,
                                              0)
         else:
-            raise ValueError, "Mode " + mode + " not supported"
+            raise IOError, "Mode " + mode + " not supported"
 
         self.fileobj = fileobj
         self.offset = 0
@@ -133,6 +133,10 @@
 
 
     def write(self,data):
+        if self.mode != WRITE:
+            import errno
+            raise IOError(errno.EBADF, "write() on read-only GzipFile object")
+        
         if self.fileobj is None:
             raise ValueError, "write() on closed GzipFile object"
         if len(data) > 0:
@@ -142,6 +146,10 @@
             self.offset += len(data)
 
     def read(self, size=-1):
+        if self.mode != READ:
+            import errno
+            raise IOError(errno.EBADF, "write() on read-only GzipFile object")
+            
         if self.extrasize <= 0 and self.fileobj is None:
             return ''