diff -r 51ac5f06dd04 -r 079aa525e506 Lib/ntpath.py
--- a/Lib/ntpath.py	Tue Jul 24 03:45:39 2012 -0700
+++ b/Lib/ntpath.py	Sat Jul 28 18:29:02 2012 +0900
@@ -339,16 +339,12 @@
         return False
     return True
 
-# Is a path a mount point?  Either a root (with or without drive letter)
-# or an UNC path with at most a / or \ after the mount point.
-
+from nt import _getvolumepathname
 def ismount(path):
-    """Test whether a path is a mount point (defined as root of drive)"""
-    seps = _get_bothseps(path)
-    root, rest = splitdrive(path)
-    if root and root[0] in seps:
-        return (not rest) or (rest in seps)
-    return rest in seps
+    """Test whether a path is a mount point"""
+    path = path.replace(altsep, sep)
+    volume = _getvolumepathname(path)
+    return volume == path
 
 
 # Expand paths beginning with '~' or '~user'.
diff -r 51ac5f06dd04 -r 079aa525e506 Lib/test/test_ntpath.py
--- a/Lib/test/test_ntpath.py	Tue Jul 24 03:45:39 2012 -0700
+++ b/Lib/test/test_ntpath.py	Sat Jul 28 18:29:02 2012 +0900
@@ -256,6 +256,19 @@
                     # dialogs (#4804)
                     ntpath.sameopenfile(-1, -1)
 
+    def test_ismount(self):
+        self.assertTrue(ntpath.ismount("c:\\"))
+        self.assertTrue(ntpath.ismount("C:\\"))
+        self.assertTrue(ntpath.ismount("c:/"))
+        self.assertTrue(ntpath.ismount("C:/"))
+        self.assertTrue(ntpath.ismount("\\\\.\\c:\\"))
+        self.assertTrue(ntpath.ismount("\\\\.\\C:\\"))
+
+        self.assertFalse(ntpath.ismount("c:"))
+        self.assertFalse(ntpath.ismount("C:"))
+        self.assertFalse(ntpath.ismount("\\\\.\\c:"))
+        self.assertFalse(ntpath.ismount(support.TESTFN))
+
 
 class NtCommonTest(test_genericpath.CommonTest):
     pathmodule = ntpath
diff -r 51ac5f06dd04 -r 079aa525e506 Modules/posixmodule.c
--- a/Modules/posixmodule.c	Tue Jul 24 03:45:39 2012 -0700
+++ b/Modules/posixmodule.c	Sat Jul 28 18:29:02 2012 +0900
@@ -3824,6 +3824,47 @@
     else
         Py_RETURN_FALSE;
 }
+
+PyDoc_STRVAR(posix__getvolumepathname__doc__,
+"Return volume mount point of the specified path.");
+
+/* A helper function for ismount on windows */
+static PyObject *
+posix__getvolumepathname(PyObject *self, PyObject *args)
+{
+    PyObject *po, *result;
+    wchar_t *path, *mountpath=NULL;
+    size_t bufsize;
+    BOOL ret;
+
+    if (!PyArg_ParseTuple(args, "U|:_getvolumepathname", &po))
+        return NULL;
+    path = PyUnicode_AsUnicode(po);
+    if (path == NULL)
+        return NULL;
+
+    /* Volume path should be shorter than entire path */
+    bufsize = max(MAX_PATH, wcslen(path) * 2 * sizeof(wchar_t)+1);
+    mountpath = malloc(bufsize);
+    if (mountpath == NULL)
+        return PyErr_NoMemory();
+
+    Py_BEGIN_ALLOW_THREADS
+    ret = GetVolumePathNameW(path, mountpath, bufsize);
+    Py_END_ALLOW_THREADS
+
+    if (!ret) {
+        result = win32_error_object("_getvolumepathname", po);
+        goto exit;
+    }
+    result = PyUnicode_FromWideChar(mountpath, wcslen(mountpath));
+exit:
+    free(mountpath);
+    return result;
+}
+/* end of posix__getvolumepathname */
+
+
 #endif /* MS_WINDOWS */
 
 PyDoc_STRVAR(posix_mkdir__doc__,
@@ -11272,6 +11313,7 @@
     {"_getfileinformation",     posix__getfileinformation, METH_VARARGS, NULL},
     {"_isdir",                  posix__isdir, METH_VARARGS, posix__isdir__doc__},
     {"_getdiskusage",           win32__getdiskusage, METH_VARARGS, win32__getdiskusage__doc__},
+    {"_getvolumepathname",      posix__getvolumepathname, METH_VARARGS, posix__getvolumepathname__doc__},
 #endif
 #ifdef HAVE_GETLOADAVG
     {"getloadavg",      posix_getloadavg, METH_NOARGS, posix_getloadavg__doc__},