diff -u b/core/lib/Drupal/Core/DependencyInjection/Compiler/RegisterSerializationClassesPass.php b/core/lib/Drupal/Core/DependencyInjection/Compiler/RegisterSerializationClassesPass.php
--- b/core/lib/Drupal/Core/DependencyInjection/Compiler/RegisterSerializationClassesPass.php
+++ b/core/lib/Drupal/Core/DependencyInjection/Compiler/RegisterSerializationClassesPass.php
@@ -45,15 +45,18 @@
   }
 
   /**
-   * Sort by priority.
+   * Sorts by priority.
    *
-   * The highest priority number is the highest priority (reverse sorting).
+   * Order services from highest priority number to lowest (reverse sorting).
    *
    * @param array $services
-   *   A nested array of Normalizers/Encoders.
+   *   A nested array keyed on priority number. For each priority number, the
+   *   value is an array of Symfony\Component\DependencyInjection\Reference
+   *   objects, each a reference to a normalizer or encoder service.
    *
    * @return array
-   *   An array of Normalizers/Encoders to be used by Serializer.
+   *   A flattened array of Reference objects from $services, ordered from high
+   *   to low priority.
    */
   protected function sort($services) {
     $sorted = array();
diff -u b/core/modules/system/lib/Drupal/system/Tests/Serialization/SerializationTest.php b/core/modules/system/lib/Drupal/system/Tests/Serialization/SerializationTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Serialization/SerializationTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Serialization/SerializationTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\system\Tests\Serialization;
 
 use Drupal\simpletest\WebTestBase;
+use Symfony\Component\Serializer\Exception\UnexpectedValueException;
 
 class SerializationTest extends WebTestBase {
 
@@ -18,6 +19,13 @@
    */
   public static $modules = array('serialization_test');
 
+  /**
+   * The serializer service to test.
+   *
+   * @var Symfony\Component\Serializer\SerializerInterface
+   */
+  protected $serializer;
+
   public static function getInfo() {
     return array(
       'name' => 'Serialization tests',
@@ -27,20 +35,38 @@
   }
 
+  protected function setUp() {
+    parent::setUp();
+    $this->serializer = drupal_container()->get('serializer');
+  }
+
   /**
    * Confirms that modules can register normalizers and encoders.
    */
   public function testSerilizerComponentRegistration() {
-    $obj = (object) array();
+    $object = new \stdClass();
+    $object->foo = 'bar';
     $format = 'serialization_test';
-    $container = drupal_container();
-    $serializer = $container->get('serializer');
+    $expected_normalization = array('foo' => 'bar', 'normalized_by' => 'SerializationTestNormalizer');
+    $expected_encoding = 'Normalized by SerializationTestNormalizer, Encoded by SerializationTestEncoder';
 
-    // Test that normalizer is added. The supportsNormalization function cannot
-    // be used to check this because if a default Normalizer is added, it will
-    // respond to any format, including the 'serialization_test' format.
-    $normalized = $serializer->normalize($obj, $format);
-    $this->assertEqual($normalized[0], 'normalize works', 'Registered normalizers are added to Serializer.');
-    // Test that encoder is added.
-    $this->assertTrue($serializer->supportsEncoding($format), 'Registered encoders are added to Serializer.');
+    // Ensure the expected normalizer and encoder get invoked.
+    $normalized = $this->serializer->normalize($object, $format);
+    $this->assertIdentical($normalized, $expected_normalization);
+    $this->assertIdentical($this->serializer->encode($normalized, $format), $expected_encoding);
+
+    // Although redundant with Symfony tests, also ensure the serialize() method
+    // correctly normalizes and then encodes.
+    $this->assertIdentical($this->serializer->serialize($object, $format), $expected_encoding);
+
+    // Ensure the normalizer and encoder do not get invoked for a format they do
+    // not support. For this, we test the serialize() method only and leave its
+    // implementation of format support checking as a black box.
+    $serialized = NULL;
+    try {
+      $serialized = $this->serializer->serialize($object, 'unsupported_format');
+    }
+    catch (UnexpectedValueException $e) {
+    }
+    $this->assertNotIdentical($serialized, $expected_encoding);
   }
 }
diff -u b/core/modules/system/tests/modules/serilization_test/lib/Drupal/serialization_test/SerializationTestEncoder.php b/core/modules/system/tests/modules/serilization_test/lib/Drupal/serialization_test/SerializationTestEncoder.php
--- b/core/modules/system/tests/modules/serilization_test/lib/Drupal/serialization_test/SerializationTestEncoder.php
+++ b/core/modules/system/tests/modules/serilization_test/lib/Drupal/serialization_test/SerializationTestEncoder.php
@@ -19,24 +19,29 @@
   static protected $format = 'serialization_test';
 
   /**
-   * Implementation required. This function not used for testing.
+   * Encodes data into the requested format.
    *
    * @param mixed $data
    *   Data to encode.
    * @param string $format
    *   Format name.
+   *
+   * @return string
+   *   A string representation of $data in the requested format.
    */
   public function encode($data, $format) {
+    // @see Drupal\serialization_test\SerializationTestNormalizer::normalize().
+    return 'Normalized by ' . $data['normalized_by'] . ', Encoded by SerializationTestEncoder';
   }
 
   /**
-   * Check whether the request is for 'serialization_test'.
+   * Checks whether this encoder can encode to the requested format.
    *
    * @param string $format
-   *   The short name of the format returned by ContentNegotiation.
+   *   The short name of the format.
    *
    * @return bool
-   *   Returns TRUE if the encoder can handle the request.
+   *   Returns TRUE if this encoder can encode to the requested format.
    */
   public function supportsEncoding($format) {
     return static::$format === $format;
diff -u b/core/modules/system/tests/modules/serilization_test/lib/Drupal/serialization_test/SerializationTestNormalizer.php b/core/modules/system/tests/modules/serilization_test/lib/Drupal/serialization_test/SerializationTestNormalizer.php
--- b/core/modules/system/tests/modules/serilization_test/lib/Drupal/serialization_test/SerializationTestNormalizer.php
+++ b/core/modules/system/tests/modules/serilization_test/lib/Drupal/serialization_test/SerializationTestNormalizer.php
@@ -27,10 +27,15 @@
    *   Format the normalization result will be encoded as.
    *
    * @return array
-   *   An array containing expected result.
+   *   An array containing a normalized representation of $object, appropriate
+   *   for encoding to the requested format.
    */
   public function normalize($object, $format = NULL) {
-    return array('normalize works');
+    $normalized = (array) $object;
+    // Add identifying value that can be used to verify that the expected
+    // normalizer was invoked.
+    $normalized['normalized_by'] = 'SerializationTestNormalizer';
+    return $normalized;
   }
 
   /**
diff -u b/core/modules/system/tests/modules/serilization_test/serialization_test.module b/core/modules/system/tests/modules/serilization_test/serialization_test.module
--- b/core/modules/system/tests/modules/serilization_test/serialization_test.module
+++ b/core/modules/system/tests/modules/serilization_test/serialization_test.module
@@ -4,3 +4,4 @@
  * @file
- * Test serialization module.
+ * Helper module for serialization tests. This file is empty, because all
+ * implementation is in autoloaded classes.
  */