We have a client that has an employee that was in the country illegally and was recently deported to Mexico. They would like to pay out his 401(k) plan balance with 0% withholding. I know that we have an Income Tax Treaty with Mexico and 0% withholding for pensions and annuities, but I have not been able to find what documentation we must request from him in order to verify his address, etc. Thanks for any thoughts on this subject.

W-8BEN? It also needed for purposes of confirming that the individual is eligible for the treaty treatment
Form and
Instructions