 <p>This should do the trick</p> <pre><code>&lt;ObjectAnimationUsingKeyFrames x:Name="animation" Duration="0" Storyboard.TargetProperty="xmlnsAlias:VisualStateUtility.InitialState" Storyboard.TargetName="ExpanderButton"&gt; </code></pre> <p>Notice how a name is added to the animation, the parentheses are removed from the target property name, which is then prefixed with the xmlns alias from the xaml header.</p> <p>In your code behind you'll have to add this:</p> <pre><code>InitializeComponent(); Storyboard.SetTargetProperty(animation, new PropertyPath(Fully.Qualified.Namespace.VisualStateUtility.InitialState)); </code></pre> <p>Apparently this last step is required for animating custom attached properties. A real pain if you'd ask me.</p>