diff --git a/NEWS b/NEWS
index 9375910b..34b892b3 100644
--- a/NEWS
+++ b/NEWS
@@ -14,6 +14,10 @@ NEWS ( CHANGELOG and HISTORY )                                     HTMLPurifier
   information
 # Transform modules changed to Tidy modules, which offer more flexibility
   and better modularization
+# Configuration object now finalizes itself when a read operation is
+  performed on it, ensuring that its internal state stays consistent.
+  To revert this behavior, you can set the $autoFinalize member variable 
+  off, but it's not recommended.
 . Unit test for ElementDef created, ElementDef behavior modified to
   be more flexible
 . Added convenience functions for HTMLModule constructors
diff --git a/library/HTMLPurifier/Config.php b/library/HTMLPurifier/Config.php
index c94e01f6..3184b8c9 100644
--- a/library/HTMLPurifier/Config.php
+++ b/library/HTMLPurifier/Config.php
@@ -35,6 +35,17 @@ class HTMLPurifier_Config
      */
     var $css_definition;
     
+    /**
+     * Bool indicator whether or not config is finalized
+     */
+    var $finalized = false;
+    
+    /**
+     * Bool indicator whether or not to automatically finalize 
+     * the object if a read operation is done
+     */
+    var $autoFinalize = true;
+    
     /**
      * @param $definition HTMLPurifier_ConfigSchema that defines what directives
      *                    are allowed.
@@ -78,6 +89,7 @@ class HTMLPurifier_Config
      * @param $key String key
      */
     function get($namespace, $key, $from_alias = false) {
+        if (!$this->finalized && $this->autoFinalize) $this->finalize();
         if (!isset($this->def->info[$namespace][$key])) {
             trigger_error('Cannot retrieve value of undefined directive',
                 E_USER_WARNING);
@@ -96,6 +108,7 @@ class HTMLPurifier_Config
      * @param $namespace String namespace
      */
     function getBatch($namespace) {
+        if (!$this->finalized && $this->autoFinalize) $this->finalize();
         if (!isset($this->def->info[$namespace])) {
             trigger_error('Cannot retrieve undefined namespace',
                 E_USER_WARNING);
@@ -111,6 +124,7 @@ class HTMLPurifier_Config
      * @param $value Mixed value
      */
     function set($namespace, $key, $value, $from_alias = false) {
+        if ($this->isFinalized('Cannot set directive after finalization')) return;
         if (!isset($this->def->info[$namespace][$key])) {
             trigger_error('Cannot set undefined directive to value',
                 E_USER_WARNING);
@@ -164,6 +178,7 @@ class HTMLPurifier_Config
      *             called before it's been setup, otherwise won't work.
      */
     function &getHTMLDefinition($raw = false) {
+        if (!$this->finalized && $this->autoFinalize) $this->finalize();
         if (
             empty($this->html_definition) || // hasn't ever been setup
             ($raw && $this->html_definition->setup) // requesting new one
@@ -179,6 +194,7 @@ class HTMLPurifier_Config
      * Retrieves reference to the CSS definition
      */
     function &getCSSDefinition() {
+        if (!$this->finalized && $this->autoFinalize) $this->finalize();
         if ($this->css_definition === null) {
             $this->css_definition = new HTMLPurifier_CSSDefinition();
             $this->css_definition->setup($this);
@@ -192,6 +208,7 @@ class HTMLPurifier_Config
      * @param $config_array Configuration associative array
      */
     function loadArray($config_array) {
+        if ($this->isFinalized('Cannot load directives after finalization')) return;
         foreach ($config_array as $key => $value) {
             $key = str_replace('_', '.', $key);
             if (strpos($key, '.') !== false) {
@@ -213,10 +230,29 @@ class HTMLPurifier_Config
      * @param $filename Name of ini file
      */
     function loadIni($filename) {
+        if ($this->isFinalized('Cannot load directives after finalization')) return;
         $array = parse_ini_file($filename, true);
         $this->loadArray($array);
     }
     
+    /**
+     * Checks whether or not the configuration object is finalized.
+     * @param $error String error message, or false for no error
+     */
+    function isFinalized($error = false) {
+        if ($this->finalized && $error) {
+            trigger_error($error, E_USER_ERROR);
+        }
+        return $this->finalized;
+    }
+    
+    /**
+     * Finalizes a configuration object, prohibiting further change
+     */
+    function finalize() {
+        $this->finalized = true;
+    }
+    
 }
 
 ?>
diff --git a/tests/HTMLPurifier/AttrDef/HTML/IDTest.php b/tests/HTMLPurifier/AttrDef/HTML/IDTest.php
index 98fffbba..add764fd 100644
--- a/tests/HTMLPurifier/AttrDef/HTML/IDTest.php
+++ b/tests/HTMLPurifier/AttrDef/HTML/IDTest.php
@@ -66,9 +66,12 @@ class HTMLPurifier_AttrDef_HTML_IDTest extends HTMLPurifier_AttrDefHarness
         
         $this->assertDef('user_story95_alas');
         $this->assertDef('user_alas', 'user_story95_user_alas'); // !
-        
+    }
+    
+    function testLocalPrefixWithoutMainPrefix() {
         // no effect when IDPrefix isn't set
         $this->config->set('Attr', 'IDPrefix', '');
+        $this->config->set('Attr', 'IDPrefixLocal', 'story95_');
         $this->expectError('%Attr.IDPrefixLocal cannot be used unless '.
             '%Attr.IDPrefix is set');
         $this->assertDef('amherst');
diff --git a/tests/HTMLPurifier/AttrDef/URITest.php b/tests/HTMLPurifier/AttrDef/URITest.php
index f9a9ab41..8f035e32 100644
--- a/tests/HTMLPurifier/AttrDef/URITest.php
+++ b/tests/HTMLPurifier/AttrDef/URITest.php
@@ -247,13 +247,10 @@ class HTMLPurifier_AttrDef_URITest extends HTMLPurifier_AttrDefHarness
         
         $this->def = new HTMLPurifier_AttrDef_URI();
         $this->config->set('URI', 'DisableExternal', true);
+        $this->config->set('URI', 'Host', 'sub.example.com');
         
         $this->assertDef('/foobar.txt');
         $this->assertDef('http://google.com/', false);
-        $this->assertDef('http://sub.example.com/alas?foo=asd', false);
-        
-        $this->config->set('URI', 'Host', 'sub.example.com');
-        
         $this->assertDef('http://sub.example.com/alas?foo=asd');
         $this->assertDef('http://example.com/teehee', false);
         $this->assertDef('http://www.example.com/#man', false);
diff --git a/tests/HTMLPurifier/ConfigTest-finalize.ini b/tests/HTMLPurifier/ConfigTest-finalize.ini
new file mode 100644
index 00000000..81720463
--- /dev/null
+++ b/tests/HTMLPurifier/ConfigTest-finalize.ini
@@ -0,0 +1,2 @@
+[Poem]
+Meter = alexandrine
\ No newline at end of file
diff --git a/tests/HTMLPurifier/ConfigTest.php b/tests/HTMLPurifier/ConfigTest.php
index 69bbf81b..61381731 100644
--- a/tests/HTMLPurifier/ConfigTest.php
+++ b/tests/HTMLPurifier/ConfigTest.php
@@ -42,6 +42,7 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         CS::define('Element', 'Object', new stdClass(), 'mixed', 'Model representation.');
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
         
         // test default value retrieval
         $this->assertIdentical($config->get('Element', 'Abbr'), 'H');
@@ -65,6 +66,12 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         $config->set('Element', 'IsotopeNames', array(238 => 'Plutonium-238', 239 => 'Plutonium-239'));
         $config->set('Element', 'Object', false); // unmodeled
         
+        $this->expectError('Cannot set undefined directive to value');
+        $config->set('Element', 'Metal', true);
+        
+        $this->expectError('Value is of invalid type');
+        $config->set('Element', 'Radioactive', 'very');
+        
         // test value retrieval
         $this->assertIdentical($config->get('Element', 'Abbr'), 'Pu');
         $this->assertIdentical($config->get('Element', 'Name'), 'plutonium');
@@ -76,17 +83,9 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         $this->assertIdentical($config->get('Element', 'IsotopeNames'), array(238 => 'Plutonium-238', 239 => 'Plutonium-239'));
         $this->assertIdentical($config->get('Element', 'Object'), false);
         
-        // errors
-        
         $this->expectError('Cannot retrieve value of undefined directive');
         $config->get('Element', 'Metal');
         
-        $this->expectError('Cannot set undefined directive to value');
-        $config->set('Element', 'Metal', true);
-        
-        $this->expectError('Value is of invalid type');
-        $config->set('Element', 'Radioactive', 'very');
-        
     }
     
     function testEnumerated() {
@@ -108,6 +107,7 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
             'synth' => 'electronic'));
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
         
         // case sensitive
         
@@ -143,6 +143,7 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         CS::define('ReportCard', 'Absences', 0, 'int', 'How many times missing from school?');
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
         
         $config->set('ReportCard', 'English', 'B-');
         $this->assertIdentical($config->get('ReportCard', 'English'), 'B-');
@@ -158,11 +159,12 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
     
     function testAliases() {
         
-        HTMLPurifier_ConfigSchema::defineNamespace('Home', 'Sweet home.');
-        HTMLPurifier_ConfigSchema::define('Home', 'Rug', 3, 'int', 'ID.');
-        HTMLPurifier_ConfigSchema::defineAlias('Home', 'Carpet', 'Home', 'Rug');
+        CS::defineNamespace('Home', 'Sweet home.');
+        CS::define('Home', 'Rug', 3, 'int', 'ID.');
+        CS::defineAlias('Home', 'Carpet', 'Home', 'Rug');
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
         
         $this->assertIdentical($config->get('Home', 'Rug'), 3);
         
@@ -183,6 +185,7 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         CS::define('Variables', 'AngularAcceleration', 'alpha', 'string', 'In rad/s^2');
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
         
         // grab a namespace
         $this->assertIdentical(
@@ -207,6 +210,7 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         CS::define('Shortcut', 'Cut', 'x', 'istring', 'Cut text');
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
         
         $config->loadIni(dirname(__FILE__) . '/ConfigTest-loadIni.ini');
         
@@ -224,6 +228,7 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         $this->old_copy = HTMLPurifier_ConfigSchema::instance($this->old_copy);
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
         
         $def = $config->getCSSDefinition();
         $this->assertIsA($def, 'HTMLPurifier_CSSDefinition');
@@ -253,9 +258,10 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
     }
     
     function test_getCSSDefinition() {
-        $this->old_copy = HTMLPurifier_ConfigSchema::instance($this->old_copy);
+        $this->old_copy = CS::instance($this->old_copy);
         
         $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
         
         $def = $config->getCSSDefinition();
         $this->assertIsA($def, 'HTMLPurifier_CSSDefinition');
@@ -263,11 +269,11 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
     
     function test_loadArray() {
         // setup a few dummy namespaces/directives for our testing
-        HTMLPurifier_ConfigSchema::defineNamespace('Zoo', 'Animals we have.');
-        HTMLPurifier_ConfigSchema::define('Zoo', 'Aadvark', 0, 'int', 'Have?');
-        HTMLPurifier_ConfigSchema::define('Zoo', 'Boar',    0, 'int', 'Have?');
-        HTMLPurifier_ConfigSchema::define('Zoo', 'Camel',   0, 'int', 'Have?');
-        HTMLPurifier_ConfigSchema::define(
+        CS::defineNamespace('Zoo', 'Animals we have.');
+        CS::define('Zoo', 'Aadvark', 0, 'int', 'Have?');
+        CS::define('Zoo', 'Boar',    0, 'int', 'Have?');
+        CS::define('Zoo', 'Camel',   0, 'int', 'Have?');
+        CS::define(
             'Zoo', 'Others', array(), 'list', 'Other animals we have one of.'
         );
         
@@ -305,9 +311,9 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
     
     function test_create() {
         
-        HTMLPurifier_ConfigSchema::defineNamespace('Cake', 'Properties of it.');
-        HTMLPurifier_ConfigSchema::define('Cake', 'Sprinkles', 666, 'int', 'Number of.');
-        HTMLPurifier_ConfigSchema::define('Cake', 'Flavor', 'vanilla', 'string', 'Flavor of the batter.');
+        CS::defineNamespace('Cake', 'Properties of it.');
+        CS::define('Cake', 'Sprinkles', 666, 'int', 'Number of.');
+        CS::define('Cake', 'Flavor', 'vanilla', 'string', 'Flavor of the batter.');
         
         $config = HTMLPurifier_Config::createDefault();
         $config->set('Cake', 'Sprinkles', 42);
@@ -326,6 +332,31 @@ class HTMLPurifier_ConfigTest extends UnitTestCase
         
     }
     
+    function test_finalize() {
+        
+        // test finalization
+        
+        CS::defineNamespace('Poem', 'Violets are red, roses are blue...');
+        CS::define('Poem', 'Meter', 'iambic', 'string', 'Rhythm of poem.');
+        
+        $config = HTMLPurifier_Config::createDefault();
+        $config->autoFinalize = false;
+        
+        $config->set('Poem', 'Meter', 'irregular');
+        
+        $config->finalize();
+        
+        $this->expectError('Cannot set directive after finalization');
+        $config->set('Poem', 'Meter', 'vedic');
+        
+        $this->expectError('Cannot load directives after finalization');
+        $config->loadArray(array('Poem.Meter' => 'octosyllable'));
+        
+        $this->expectError('Cannot load directives after finalization');
+        $config->loadIni(dirname(__FILE__) . '/ConfigTest-finalize.ini');
+        
+    }
+    
 }
 
 ?>
\ No newline at end of file
diff --git a/tests/HTMLPurifier/EncoderTest.php b/tests/HTMLPurifier/EncoderTest.php
index ef14b139..0bab873e 100644
--- a/tests/HTMLPurifier/EncoderTest.php
+++ b/tests/HTMLPurifier/EncoderTest.php
@@ -39,7 +39,9 @@ class HTMLPurifier_EncoderTest extends UnitTestCase
         );
         $this->assertNoErrors();
         
-        $config->set('Core', 'Encoding', 'ISO-8859-1');
+        $config = HTMLPurifier_Config::create(array(
+            'Core.Encoding' => 'ISO-8859-1'
+        ));
         
         // Now it gets converted
         $this->assertIdentical(
@@ -47,8 +49,10 @@ class HTMLPurifier_EncoderTest extends UnitTestCase
             "\xC3\xB6"
         );
         
-        $config->set('Test', 'ForceNoIconv', true);
-        
+        $config = HTMLPurifier_Config::create(array(
+            'Core.Encoding' => 'ISO-8859-1',
+            'Test.ForceNoIconv' => true
+        ));
         $this->assertIdentical(
             HTMLPurifier_Encoder::convertToUTF8("\xF6", $config, $context),
             "\xC3\xB6"
@@ -69,7 +73,9 @@ class HTMLPurifier_EncoderTest extends UnitTestCase
             "\xC3\xB6"
         );
         
-        $config->set('Core', 'Encoding', 'ISO-8859-1');
+        $config = HTMLPurifier_Config::create(array(
+            'Core.Encoding' => 'ISO-8859-1'
+        ));
         
         // Now it gets converted
         $this->assertIdentical(
@@ -86,7 +92,10 @@ class HTMLPurifier_EncoderTest extends UnitTestCase
         }
         
         // Plain PHP implementation has slightly different behavior
-        $config->set('Test', 'ForceNoIconv', true);
+        $config = HTMLPurifier_Config::create(array(
+            'Core.Encoding' => 'ISO-8859-1',
+            'Test.ForceNoIconv' => true
+        ));
         $this->assertIdentical(
             HTMLPurifier_Encoder::convertFromUTF8("\xC3\xB6", $config, $context),
             "\xF6"
@@ -98,8 +107,10 @@ class HTMLPurifier_EncoderTest extends UnitTestCase
         );
         
         // Preserve the characters!
-        
-        $config->set('Core', 'EscapeNonASCIICharacters', true);
+        $config = HTMLPurifier_Config::create(array(
+            'Core.Encoding' => 'ISO-8859-1',
+            'Core.EscapeNonASCIICharacters' => true
+        ));
         $this->assertIdentical(
             HTMLPurifier_Encoder::convertFromUTF8($chinese, $config, $context),
             "中文 (Chinese)"
diff --git a/tests/HTMLPurifier/URISchemeRegistryTest.php b/tests/HTMLPurifier/URISchemeRegistryTest.php
index 21a24348..8edc3bdc 100644
--- a/tests/HTMLPurifier/URISchemeRegistryTest.php
+++ b/tests/HTMLPurifier/URISchemeRegistryTest.php
@@ -9,9 +9,10 @@ class HTMLPurifier_URISchemeRegistryTest extends UnitTestCase
         
         generate_mock_once('HTMLPurifier_URIScheme');
         
-        $config = HTMLPurifier_Config::createDefault();
-        $config->set('URI', 'AllowedSchemes', array('http' => true, 'telnet' => true));
-        $config->set('URI', 'OverrideAllowedSchemes', true);
+        $config = HTMLPurifier_Config::create(array(
+            'URI.AllowedSchemes' => 'http, telnet',
+            'URI.OverrideAllowedSchemes' => true
+        ));
         $context = new HTMLPurifier_Context();
         
         $registry = new HTMLPurifier_URISchemeRegistry();
@@ -34,7 +35,10 @@ class HTMLPurifier_URISchemeRegistryTest extends UnitTestCase
         $this->assertIdentical($registry->getScheme('foobar', $config, $context), $scheme_foobar);
         
         // now, test when overriding is not allowed
-        $config->set('URI', 'OverrideAllowedSchemes', false);
+        $config = HTMLPurifier_Config::create(array(
+            'URI.AllowedSchemes' => 'http, telnet',
+            'URI.OverrideAllowedSchemes' => false
+        ));
         $this->assertNull($registry->getScheme('foobar', $config, $context));
         
         // scheme not allowed and never registered