diff --git a/Lib/ConfigParser.py b/Lib/ConfigParser.py
index 65c8ce5..2902939 100644
--- a/Lib/ConfigParser.py
+++ b/Lib/ConfigParser.py
@@ -594,7 +594,8 @@
         self._interpolate_some(option, L, rawval, section, vars, 1)
         return ''.join(L)
 
-    _interpvar_match = re.compile(r"%\(([^)]+)\)s").match
+    _interpvar_re = re.compile(r"%\(([^)]+)\)s")
+    _badpercent_re = re.compile(r"%[^%]|%$")
 
     def _interpolate_some(self, option, accum, rest, section, map, depth):
         if depth > MAX_INTERPOLATION_DEPTH:
@@ -613,7 +614,7 @@
                 accum.append("%")
                 rest = rest[2:]
             elif c == "(":
-                m = self._interpvar_match(rest)
+                m = self._interpvar_re.match(rest)
                 if m is None:
                     raise InterpolationSyntaxError(option, section,
                         "bad interpolation variable reference %r" % rest)
@@ -638,4 +639,12 @@
         """Set an option.  Extend ConfigParser.set: check for string values."""
         if not isinstance(value, basestring):
             raise TypeError("option values must be strings")
+        # check for bad percent signs:
+        # first, replace all "good" interpolations
+        tmp_value = self._interpvar_re.sub('', value)
+        # then, check if there's a lone percent sign left
+        m = self._badpercent_re.search(tmp_value)
+        if m:
+            raise ValueError("invalid interpolation syntax in %r at "
+                             "position %d" % (value, m.start()))
         ConfigParser.set(self, section, option, value)