diff --git a/Lib/test/test_minidom.py b/Lib/test/test_minidom.py
index 446bbe096bd19d..ee31fd24bdaec4 100644
--- a/Lib/test/test_minidom.py
+++ b/Lib/test/test_minidom.py
@@ -6,6 +6,7 @@
 from test import support
 import unittest
 
+import xml.dom.expatbuilder
 import xml.dom.minidom
 
 from xml.dom.minidom import parse, Attr, Node, Document, Element, parseString
@@ -683,6 +684,54 @@ def testWriteXMLExistingNamespaceDeclarations(self):
                 self.assertEqual(dom.documentElement.toxml(), str)
                 dom.unlink()
 
+    def testWriteXMLDOM1PrefixDeclaration(self):
+        # setAttribute() does not set the prefix of the attribute,
+        # but "xmlns:p" still declares the prefix "p".
+        dom = Document()
+        root = dom.appendChild(
+            dom.createElementNS("http://xml.python.org/ns", "p:root"))
+        root.setAttribute("xmlns:p", "http://xml.python.org/ns")
+        domstr = dom.documentElement.toxml()
+        dom.unlink()
+        self.assertEqual(domstr,
+                '<p:root xmlns:p="http://xml.python.org/ns"/>')
+        dom = parseString(domstr)
+        self.assertEqual(dom.documentElement.namespaceURI,
+                         "http://xml.python.org/ns")
+        dom.unlink()
+
+    def testWriteXMLDOM1PrefixDeclarationNoNamespace(self):
+        # A prefix declaration is not a declaration of the default namespace.
+        dom = Document()
+        root = dom.appendChild(dom.createElement("root"))
+        root.setAttribute("xmlns:xsi", "http://xml.python.org/ns")
+        self.assertEqual(dom.documentElement.toxml(),
+                '<root xmlns:xsi="http://xml.python.org/ns"/>')
+        dom.unlink()
+
+    def testWriteXMLDOM1PrefixDeclarationDefaultNamespace(self):
+        # The element still needs a declaration of its own default namespace.
+        dom = Document()
+        root = dom.appendChild(
+            dom.createElementNS("http://xml.python.org/ns", "root"))
+        root.setAttribute("xmlns:x", "http://xml.python.org/ns")
+        domstr = dom.documentElement.toxml()
+        dom.unlink()
+        self.assertEqual(domstr,
+                '<root xmlns="http://xml.python.org/ns" '
+                'xmlns:x="http://xml.python.org/ns"/>')
+        dom = parseString(domstr)
+        self.assertEqual(dom.documentElement.namespaceURI,
+                         "http://xml.python.org/ns")
+        dom.unlink()
+
+    def testWriteXMLParsedWithoutNamespaces(self):
+        src = ('<root xmlns:p="http://xml.python.org/ns">'
+               '<p:child/><other/></root>')
+        dom = xml.dom.expatbuilder.parseString(src, namespaces=False)
+        self.assertEqual(dom.documentElement.toxml(), src)
+        dom.unlink()
+
     def testWriteXMLNotANamespaceDeclaration(self):
         # an attribute whose name only starts with "xmlns" is not one
         dom = parseString('<root xmlns="http://xml.python.org/ns">'
diff --git a/Lib/xml/dom/minidom.py b/Lib/xml/dom/minidom.py
index 7cb652a323dcc2..253934eecfaaa5 100644
--- a/Lib/xml/dom/minidom.py
+++ b/Lib/xml/dom/minidom.py
@@ -433,7 +433,7 @@ def _fixup_namespaces(element, nsmap):
                 # Declarations already present in the document take precedence.
                 nsmap = _bind_namespace(
                     nsmap, inherited,
-                    attr.localName if attr.prefix else None, attr.value)
+                    attr.localName if ":" in name else None, attr.value)
                 attr_uri = None
             elif attr_uri == XML_NAMESPACE:
                 # The xml prefix is bound by definition.