diff --git a/Lib/test/test_xml_etree.py b/Lib/test/test_xml_etree.py
--- a/Lib/test/test_xml_etree.py
+++ b/Lib/test/test_xml_etree.py
@@ -1612,7 +1612,37 @@
         ET.register_namespace('test10777', 'http://myuri/')
         ET.register_namespace('test10777', 'http://myuri/')
 
-
+    def test_pi_treatment(self):
+        root = ET.Element('body', { 'text': 'some text'})
+        root.append(ET.ProcessingInstruction('verb', 'extra data'))
+        tree = ET.ElementTree(root)
+        stream = io.BytesIO()
+        tree.write(stream)
+
+        # PIs are serialized to XML..
+        self.assertTrue(b'<?verb extra data?>' in bytes(stream.getbuffer())) 
+
+        # ..but skipped when parsing XML
+        stream.seek(0)
+        tree2 = ET.parse(stream)
+        self.assertTrue(len(tree2.getroot().getchildren()) == 0)
+
+
+    def test_comment_treatment(self):
+        root = ET.Element('body', { 'text': 'some text'})
+        root.append(ET.Comment('not to be rendered'))
+        tree = ET.ElementTree(root)
+        stream = io.BytesIO()
+        tree.write(stream)
+
+        # PIs are serialized to XML..
+        self.assertTrue(b'<!--not to be rendered-->' in bytes(stream.getbuffer())) 
+
+        # ..but skipped when parsing XML
+        stream.seek(0)
+        tree2 = ET.parse(stream)
+        self.assertTrue(len(tree2.getroot().getchildren()) == 0)
+        
 # --------------------------------------------------------------------