diff --git a/NEWS b/NEWS
index 7255569c..f01b4f67 100644
--- a/NEWS
+++ b/NEWS
@@ -10,6 +10,7 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
 
 1.2.0, unknown projected release date
 ! Added MODx plugin <http://modxcms.com/forums/index.php/topic,6604.0.html>
+! Added percent encoding normalization
 - Documentation updated
   + TODO added request Phalanger
   + TODO added request Native compression
diff --git a/library/HTMLPurifier/AttrDef/URI.php b/library/HTMLPurifier/AttrDef/URI.php
index c5181104..770b1282 100644
--- a/library/HTMLPurifier/AttrDef/URI.php
+++ b/library/HTMLPurifier/AttrDef/URI.php
@@ -4,6 +4,7 @@ require_once 'HTMLPurifier/AttrDef.php';
 require_once 'HTMLPurifier/URIScheme.php';
 require_once 'HTMLPurifier/URISchemeRegistry.php';
 require_once 'HTMLPurifier/AttrDef/Host.php';
+require_once 'HTMLPurifier/PercentEncoder.php';
 
 HTMLPurifier_ConfigSchema::define(
     'URI', 'DefaultScheme', 'http', 'string',
@@ -19,9 +20,11 @@ class HTMLPurifier_AttrDef_URI extends HTMLPurifier_AttrDef
 {
     
     var $host;
+    var $PercentEncoder;
     
     function HTMLPurifier_AttrDef_URI() {
         $this->host = new HTMLPurifier_AttrDef_Host();
+        $this->PercentEncoder = new HTMLPurifier_PercentEncoder();
     }
     
     function validate($uri, $config, &$context) {
@@ -32,6 +35,9 @@ class HTMLPurifier_AttrDef_URI extends HTMLPurifier_AttrDef
         // parse as CDATA
         $uri = $this->parseCDATA($uri);
         
+        // fix up percent-encoding
+        $uri = $this->PercentEncoder->normalize($uri);
+        
         // while it would be nice to use parse_url(), that's specifically
         // for HTTP and thus won't work for our generic URI parsing
         
diff --git a/library/HTMLPurifier/PercentEncoder.php b/library/HTMLPurifier/PercentEncoder.php
new file mode 100644
index 00000000..f5f26527
--- /dev/null
+++ b/library/HTMLPurifier/PercentEncoder.php
@@ -0,0 +1,44 @@
+<?php
+
+class HTMLPurifier_PercentEncoder
+{
+    
+    /**
+     * Fix up percent-encoding by decoding unreserved characters and normalizing
+     * @param $string String to normalize
+     */
+    function normalize($string) {
+        if ($string == '') return '';
+        $parts = explode('%', $string);
+        $ret = array_shift($parts);
+        foreach ($parts as $part) {
+            $length = strlen($part);
+            if ($length < 2) {
+                $ret .= '%25' . $part;
+                continue;
+            }
+            $encoding = substr($part, 0, 2);
+            $text     = substr($part, 2);
+            if (!ctype_xdigit($encoding)) {
+                $ret .= '%25' . $part;
+                continue;
+            }
+            $int = hexdec($encoding);
+            if (
+                ($int >= 48 && $int <= 57) || // digits
+                ($int >= 65 && $int <= 90) || // uppercase letters
+                ($int >= 97 && $int <= 122) || // lowercase letters
+                $int == 126 || $int == 45 || $int == 46 || $int == 95 // ~-._
+            ) {
+                $ret .= chr($int) . $text;
+                continue;
+            }
+            $encoding = strtoupper($encoding);
+            $ret .= '%' . $encoding . $text;
+        }
+        return $ret;
+    }
+    
+}
+
+?>
\ No newline at end of file
diff --git a/tests/HTMLPurifier/AttrDef/URITest.php b/tests/HTMLPurifier/AttrDef/URITest.php
index cec094ec..8b178ebc 100644
--- a/tests/HTMLPurifier/AttrDef/URITest.php
+++ b/tests/HTMLPurifier/AttrDef/URITest.php
@@ -4,7 +4,6 @@ require_once 'HTMLPurifier/AttrDefHarness.php';
 require_once 'HTMLPurifier/AttrDef/URI.php';
 
 // WARNING: INCOMPLETE UNIT TESTS!
-// we are currently abstaining percent-encode fixing unit tests
 // we also need to test all the configuration directives defined by this class
 
 // http: is returned quite often when a URL is invalid. We have to change
@@ -83,10 +82,11 @@ class HTMLPurifier_AttrDef_URITest extends HTMLPurifier_AttrDefHarness
         // %5 - prematurely terminated, encode %
         // %FC - u with umlaut, correct
         // note that Apache doesn't do such fixing, rather, it just claims
-        // that the browser sent a "Bad Request".
-        //$uri[6] = 'http://www.example.com/%56%fc%GJ%5%FC';
-        //$components[6] = array('www.example.com', '/V%FC%25GJ%255%FC', null, null);
-        //$expect_uri[6] = 'http://www.example.com/V%FC%25GJ%255%FC';
+        // that the browser sent a "Bad Request".  See PercentEncoder.php
+        // for more details
+        $uri[6] = 'http://www.example.com/%56%fc%GJ%5%FC';
+        $components[6] = array(null, 'www.example.com', null, '/V%FC%25GJ%255%FC', null);
+        $expect_uri[6] = 'http://www.example.com/V%FC%25GJ%255%FC';
         
         // test IPv4 address (behavior may vary with configuration)
         $uri[7] = 'http://192.0.34.166/';
diff --git a/tests/HTMLPurifier/PercentEncoderTest.php b/tests/HTMLPurifier/PercentEncoderTest.php
new file mode 100644
index 00000000..ef5ac869
--- /dev/null
+++ b/tests/HTMLPurifier/PercentEncoderTest.php
@@ -0,0 +1,42 @@
+<?php
+
+require_once 'HTMLPurifier/PercentEncoder.php';
+
+class HTMLPurifier_PercentEncoderTest extends UnitTestCase
+{
+    
+    var $PercentEncoder;
+    var $func;
+    
+    function setUp() {
+        $this->PercentEncoder = new HTMLPurifier_PercentEncoder();
+        $this->func = '';
+    }
+    
+    function assertDecode($string, $expect = true) {
+        if ($expect === true) $expect = $string;
+        $this->assertEqual($this->PercentEncoder->{$this->func}($string), $expect);
+    }
+    
+    function test_normalize() {
+        $this->func = 'normalize';
+        
+        $this->assertDecode('Aw.../-$^8'); // no change
+        $this->assertDecode('%41%77%7E%2D%2E%5F', 'Aw~-._'); // decode unreserved chars
+        $this->assertDecode('%3A%2F%3F%23%5B%5D%40%21%24%26%27%28%29%2A%2B%2C%3B%3D'); // preserve reserved chars
+        $this->assertDecode('%2b', '%2B'); // normalize to uppercase
+        $this->assertDecode('%2B2B%3A3A'); // extra text
+        $this->assertDecode('%2b2B%4141', '%2B2BA41'); // extra text, with normalization
+        $this->assertDecode('%', '%25'); // normalize stray percent sign
+        $this->assertDecode('%5%25', '%255%25'); // permaturely terminated encoding
+        $this->assertDecode('%GJ', '%25GJ'); // invalid hexadecimal chars
+        
+        // contested behavior, if this changes, we'll also have to have
+        // outbound encoding
+        $this->assertDecode('%FC'); // not reserved or unreserved, preserve
+        
+    }
+    
+}
+
+?>
\ No newline at end of file
diff --git a/tests/index.php b/tests/index.php
index 0c898af1..4ceb8d96 100644
--- a/tests/index.php
+++ b/tests/index.php
@@ -90,6 +90,7 @@ $test_files[] = 'EncoderTest.php';
 $test_files[] = 'EntityParserTest.php';
 $test_files[] = 'Test.php';
 $test_files[] = 'ContextTest.php';
+$test_files[] = 'PercentEncoderTest.php';
 
 if (version_compare(PHP_VERSION, '5', '>=')) {
     $test_files[] = 'TokenFactoryTest.php';