diff --git a/src/xmldiff/parser.py b/src/xmldiff/parser.py
index 93e4069..b3fd627 100644
--- a/src/xmldiff/parser.py
+++ b/src/xmldiff/parser.py
@@ -71,9 +71,13 @@ def endPrefixMapping(self, prefix):
     def _buildTag(self, ns_name_tuple):
         ns_uri, local_name = ns_name_tuple
         if ns_uri:
-            el_tag = "{%s}%s" % ns_name_tuple
+            ns_name = [x[0] for x in self._ns_mapping.items()
+                       if ns_uri in x[1]][0]
+            el_tag = "%s:%s" % (ns_name, local_name)
         elif self._default_ns:
-            el_tag = "{%s}%s" % (self._default_ns, local_name)
+            ns_name = [x[0] for x in self._ns_mapping.items()
+                       if self._default_ns in x[1]][0]
+            el_tag = "%s:%s" % (ns_name, local_name)
         else:
             el_tag = local_name
         return el_tag
diff --git a/tests/data/test10_ns_result b/tests/data/test10_ns_result
index 24aa9ee..46b2c05 100644
--- a/tests/data/test10_ns_result
+++ b/tests/data/test10_ns_result
@@ -38,7 +38,7 @@ Insert mixed element with text
 [rename, /Tests[1]/Test[7]/Eight[1], Six]
 [move-first, /Tests[1]/Test[6]/@type, /Tests[1]/Test[8]]
 [move-first, /Tests[1]/Test[7]/@type, /Tests[1]/Test[9]]
-[update, /Tests[1]/Test[3]/{urn:test_ns}Four[1]/text()[1], This WAS the fourth sentence.]
+[update, /Tests[1]/Test[3]/tns:Four[1]/text()[1], This WAS the fourth sentence.]
 [update, /Tests[1]/Test[6]/Five[1]/text()[1], This is the]
 [insert-after, /Tests[1]/Test[6]/Five[1]/text()[1],
 <new/>
@@ -57,7 +57,7 @@ and improved
 [append-first, /Tests[1]/Test[10]/Nine[1]/b[1],
 (changed)
 ]
-[remove, /Tests[1]/{urn:test_ns}Test[1]/Two[2]]
+[remove, /Tests[1]/tns:Test[1]/Two[2]]
 [remove, /Tests[1]/Test[2]/Three[1]]
 [remove, /Tests[1]/Test[4]]
 [remove, /Tests[1]/Test[4]]
diff --git a/tests/data/test11_ns_result b/tests/data/test11_ns_result
index 1968be9..8af773c 100644
--- a/tests/data/test11_ns_result
+++ b/tests/data/test11_ns_result
@@ -1 +1 @@
-[update, /Tests[1]/Test[1]/Nine[1]/b[1]/@{urn:test_ns}name, NineNameChanged]
+[update, /Tests[1]/Test[1]/Nine[1]/b[1]/@tns:name, NineNameChanged]
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 3fbc920..f7ff875 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -150,21 +150,21 @@ def test_tree_from_stream_with_namespace():
         '/',
         '',
         [[1,
-          u'{urn:corp:sec}section',
-          u'{urn:corp:sec}section',
+          u'sec:section',
+          u'sec:section',
           [[1,
-            u'{urn:corp:sec}sectionInfo',
-            u'{urn:corp:sec}sectionInfo',
+            u'sec:sectionInfo',
+            u'sec:sectionInfo',
             [[1,
-              u'{urn:corp:sec}secID',
-              u'{urn:corp:sec}secID',
+              u'sec:secID',
+              u'sec:secID',
               [[4, 'text()', u'S001', [], None, 0, 1]],
               None,
               1,
               1],
              [1,
-              u'{urn:corp:sec}name',
-              u'{urn:corp:sec}name',
+              u'sec:name',
+              u'sec:name',
               [[4, 'text()', u'Sales', [], None, 0, 1]],
               None,
               1,
@@ -173,8 +173,8 @@ def test_tree_from_stream_with_namespace():
             4,
             1],
            [1,
-            u'{urn:corp:sec}sectionInfo',
-            u'{urn:corp:sec}sectionInfo',
+            u'sec:sectionInfo',
+            u'sec:sectionInfo',
             [[2,
               u'@nameName',
               u'name',
@@ -193,19 +193,19 @@ def test_tree_from_stream_with_namespace():
             4,
             2],
            [1,
-            u'{urn:corp:sec}sectionInfo',
-            u'{urn:corp:sec}sectionInfo',
+            u'sec:sectionInfo',
+            u'sec:sectionInfo',
             [[2,
-              u'@{urn:corp:sec}nameName',
-              u'{urn:corp:sec}name',
-              [[3, u'@{urn:corp:sec}name', u'Gardening', [], None, 0, 0]],
+              u'@sec:nameName',
+              u'sec:name',
+              [[3, u'@sec:name', u'Gardening', [], None, 0, 0]],
               None,
               1,
               0],
              [2,
-              u'@{urn:corp:sec}secIDName',
-              u'{urn:corp:sec}secID',
-              [[3, u'@{urn:corp:sec}secID', u'S003', [], None, 0, 0]],
+              u'@sec:secIDName',
+              u'sec:secID',
+              [[3, u'@sec:secID', u'S003', [], None, 0, 0]],
               None,
               1,
               0]],
@@ -249,6 +249,22 @@ def test_tree_from_lxml_with_namespace():
     _nuke_parent(tree)
     _nuke_parent(tree_stream)
 
+    # In lxml, up to and including version 4.2.1, the namespace prefixes
+    # will be replaced by auto-generated namespace prefixes, ns00, ns01, etc
+    # If we encounter an "ns00:"" prefix, replace it.
+    # This code can be removed once we no longer need to run the tests with
+    # lxml 4.2.1 or earlier.
+    # This is only to fix this test, using xmldiff with these versions of
+    # lxml will still work, but the prefixes will be wrong.
+    def fix_tree(t, prefix):
+        t[1] = t[1].replace('ns00:', prefix)
+        t[2] = t[2].replace('ns00:', prefix)
+        for subtree in t[3]:
+            fix_tree(subtree, prefix)
+
+    # lxml <= 4.2.1
+    fix_tree(tree, 'sec:')
+
     assert tree == tree_stream
 
     fname = os.path.join(HERE, 'data', 'parse', 'tal_ns.xml')
@@ -261,6 +277,9 @@ def test_tree_from_lxml_with_namespace():
     _nuke_parent(tree)
     _nuke_parent(tree_stream)
 
+    # lxml <= 4.2.1
+    fix_tree(tree, 'z:')
+
     assert tree == tree_stream