Hi there, This error normally happens in the two common cases as below: + When you post the current page to another web page, and the next page failed to load the ViewState data. + The control hierarchy when the ViewState data is saved is not the same as the one when the ViewState is loaded. Do you dynamically add any control in code at runtime? Btw, which treeview control are you using? If you still cannot figure out the problem, you might want to post a snippet of your sample code.