diff -r 4828cb77bf2a Lib/pathlib.py
--- a/Lib/pathlib.py	Wed Feb 11 17:49:01 2015 -0800
+++ b/Lib/pathlib.py	Thu Feb 12 23:34:51 2015 +0200
@@ -73,6 +73,10 @@ class _Flavour(object):
                     # parts. This makes the result of parsing e.g.
                     # ("C:", "/", "a") reasonably intuitive.
                     for part in it:
+                        if not part:
+                            continue
+                        if altsep:
+                            part = part.replace(altsep, sep)
                         drv = self.splitroot(part)[0]
                         if drv:
                             break
diff -r 4828cb77bf2a Lib/test/test_pathlib.py
--- a/Lib/test/test_pathlib.py	Wed Feb 11 17:49:01 2015 -0800
+++ b/Lib/test/test_pathlib.py	Thu Feb 12 23:34:51 2015 +0200
@@ -102,31 +102,35 @@ class NTFlavourTest(_BaseFlavourTest, un
         check = self._check_parse_parts
         # First part is anchored
         check(['c:'],                   ('c:', '', ['c:']))
-        check(['c:\\'],                 ('c:', '\\', ['c:\\']))
-        check(['\\'],                   ('', '\\', ['\\']))
+        check(['c:/'],                  ('c:', '\\', ['c:\\']))
+        check(['/'],                    ('', '\\', ['\\']))
         check(['c:a'],                  ('c:', '', ['c:', 'a']))
-        check(['c:\\a'],                ('c:', '\\', ['c:\\', 'a']))
-        check(['\\a'],                  ('', '\\', ['\\', 'a']))
+        check(['c:/a'],                 ('c:', '\\', ['c:\\', 'a']))
+        check(['/a'],                   ('', '\\', ['\\', 'a']))
         # UNC paths
-        check(['\\\\a\\b'],             ('\\\\a\\b', '\\', ['\\\\a\\b\\']))
-        check(['\\\\a\\b\\'],           ('\\\\a\\b', '\\', ['\\\\a\\b\\']))
-        check(['\\\\a\\b\\c'],          ('\\\\a\\b', '\\', ['\\\\a\\b\\', 'c']))
+        check(['//a/b'],                ('\\\\a\\b', '\\', ['\\\\a\\b\\']))
+        check(['//a/b/'],               ('\\\\a\\b', '\\', ['\\\\a\\b\\']))
+        check(['//a/b/c'],              ('\\\\a\\b', '\\', ['\\\\a\\b\\', 'c']))
         # Second part is anchored, so that the first part is ignored
         check(['a', 'Z:b', 'c'],        ('Z:', '', ['Z:', 'b', 'c']))
-        check(['a', 'Z:\\b', 'c'],      ('Z:', '\\', ['Z:\\', 'b', 'c']))
-        check(['a', '\\b', 'c'],        ('', '\\', ['\\', 'b', 'c']))
+        check(['a', 'Z:/b', 'c'],       ('Z:', '\\', ['Z:\\', 'b', 'c']))
         # UNC paths
-        check(['a', '\\\\b\\c', 'd'],   ('\\\\b\\c', '\\', ['\\\\b\\c\\', 'd']))
+        check(['a', '//b/c', 'd'],      ('\\\\b\\c', '\\', ['\\\\b\\c\\', 'd']))
         # Collapsing and stripping excess slashes
-        check(['a', 'Z:\\\\b\\\\c\\', 'd\\'], ('Z:', '\\', ['Z:\\', 'b', 'c', 'd']))
+        check(['a', 'Z://b//c/', 'd/'], ('Z:', '\\', ['Z:\\', 'b', 'c', 'd']))
         # UNC paths
-        check(['a', '\\\\b\\c\\\\', 'd'], ('\\\\b\\c', '\\', ['\\\\b\\c\\', 'd']))
+        check(['a', '//b/c//', 'd'],    ('\\\\b\\c', '\\', ['\\\\b\\c\\', 'd']))
         # Extended paths
-        check(['\\\\?\\c:\\'],          ('\\\\?\\c:', '\\', ['\\\\?\\c:\\']))
-        check(['\\\\?\\c:\\a'],         ('\\\\?\\c:', '\\', ['\\\\?\\c:\\', 'a']))
+        check(['//?/c:/'],              ('\\\\?\\c:', '\\', ['\\\\?\\c:\\']))
+        check(['//?/c:/a'],             ('\\\\?\\c:', '\\', ['\\\\?\\c:\\', 'a']))
+        check(['//?/c:/a', '/b'],       ('\\\\?\\c:', '\\', ['\\\\?\\c:\\', 'b']))
         # Extended UNC paths (format is "\\?\UNC\server\share")
-        check(['\\\\?\\UNC\\b\\c'],     ('\\\\?\\UNC\\b\\c', '\\', ['\\\\?\\UNC\\b\\c\\']))
-        check(['\\\\?\\UNC\\b\\c\\d'],  ('\\\\?\\UNC\\b\\c', '\\', ['\\\\?\\UNC\\b\\c\\', 'd']))
+        check(['//?/UNC/b/c'],          ('\\\\?\\UNC\\b\\c', '\\', ['\\\\?\\UNC\\b\\c\\']))
+        check(['//?/UNC/b/c/d'],        ('\\\\?\\UNC\\b\\c', '\\', ['\\\\?\\UNC\\b\\c\\', 'd']))
+        # Second part has a root but not drive
+        check(['a', '/b', 'c'],         ('', '\\', ['\\', 'b', 'c']))
+        check(['Z:/a', '/b', 'c'],      ('Z:', '\\', ['Z:\\', 'b', 'c']))
+        check(['//?/Z:/a', '/b', 'c'],  ('\\\\?\\Z:', '\\', ['\\\\?\\Z:\\', 'b', 'c']))
 
     def test_splitroot(self):
         f = self.flavour.splitroot