Index: library/Zend/Json/Encoder.php
===================================================================
--- library/Zend/Json/Encoder.php       (revision 24415)
+++ library/Zend/Json/Encoder.php       (working copy)
@@ -135,22 +135,24 @@
         }

         $props = '';
-
-        if ($value instanceof Iterator) {
-            $propCollection = $value;
+        if (method_exists($value, 'toJson')) {
+            $props =',' . preg_replace("/^\{(.*)\}$/","\\1",$value->toJson());
         } else {
-            $propCollection = get_object_vars($value);
-        }
+            if ($value instanceof Iterator) {
+                $propCollection = $value;
+            } else {
+                $propCollection = get_object_vars($value);
+            }

-        foreach ($propCollection as $name => $propValue) {
-            if (isset($propValue)) {
-                $props .= ','
-                        . $this->_encodeString($name)
-                        . ':'
-                        . $this->_encodeValue($propValue);
+            foreach ($propCollection as $name => $propValue) {
+                if (isset($propValue)) {
+                    $props .= ','
+                            . $this->_encodeString($name)
+                            . ':'
+                            . $this->_encodeValue($propValue);
+                }
             }
         }
-
         $className = get_class($value);
         return '{"__className":' . $this->_encodeString($className)
                 . $props . '}';
Index: tests/Zend/JsonTest.php
===================================================================
--- tests/Zend/JsonTest.php     (revision 24415)
+++ tests/Zend/JsonTest.php     (working copy)
@@ -811,8 +811,19 @@
         $this->assertEquals($targetHtmlOutput, Zend_Json::prettyPrint($jsonstr, array('format' => 'html')));
     }

+    /**
+     * @group ZF-9521
+     */
+    public function testWillEncodeArrayOfObjectsEachWithToJsonMethod()
+    {
+        $array = array('one'=>new ToJsonClass());
+        $expected = '{"one":{"__className":"ToJsonClass","firstName":"John","lastName":"Doe","email":"john@doe.com"}}';
+
+        Zend_Json::$useBuiltinEncoderDecoder = true;
+        $json = Zend_Json::encode($array);
+        $this->assertEquals($expected, $json);
+    }

-
 }

 /**