Contacting deactivated hydrodesulfurization catalyst, such as Co-Mo supported on alumina, with a heteropoly acid, such as molybdophosphoric acid or tungstosilicic acid, to which dilute hydrogen peroxide is added, results in a significant enhancement in the rate and extent of carbon and metals removal. Forms of deposited vanadium intractable by other treatments are also removed. The process results in substantially complete regeneration of catalytic activity.