diff -u b/menu_attributes.module b/menu_attributes.module
--- b/menu_attributes.module
+++ b/menu_attributes.module
@@ -139,7 +139,7 @@
   $attributes = variable_get('menu_attributes_menus', array());
 
   // Set default attributes.
-  $default_attributes = !empty($attributes[$menu_name]) ? $attributes[$menu_name] : array('id' => '', 'class' => 'menu');
+  $default_attributes = !empty($attributes[$menu_name]) ? $attributes[$menu_name] : array('id' => '', 'class' => array('menu'));
 
   $form['attributes'] = array(
     '#type' => 'fieldset',
@@ -156,7 +156,7 @@
   $form['attributes']['class'] = array(
     '#type' => 'textfield',
     '#title' => t('Classes'),
-    '#default_value' => $default_attributes['class'],
+    '#default_value' => implode(' ', $default_attributes['class']),
   );
 
   $form['#submit'][] = '_menu_attributes_menu_form_submit';
@@ -178,7 +178,7 @@
   // Store attributes in variable.
   $attributes[$menu_name] = array(
     'id' => $values['attributes']['id'],
-    'class' => $values['attributes']['class'],
+    'class' => array_filter(explode(' ', $values['attributes']['class'])),
   );
   variable_set('menu_attributes_menus', $attributes);
 }
@@ -429,7 +429,7 @@
  */
 function menu_attributes_menu_tree($variables) {
   $menus = variable_get('menu_attributes_menus', array());
-  $attributes = array('class' => 'menu');
+  $attributes = array('class' => array('menu'));
   if (!empty($menus[$variables['menu_name']]) && $variables['depth'] == 1) {
     $attributes = array_filter($menus[$variables['menu_name']]);
   }