The structural and morphological changes of the metal phases which occur during regeneration in hydrogen and air/hydrogen cycles have been studied for Ni-USY catalysts which had been deactivated during the hydrogenation of benzene. Independent of nickel loading in the zeolite, regeneration in hydrogen alone was not sufficient to restore the initial:activity. Regeneration in air/hydrogen cycles was more successful for catalysts with higher Ni loadings, but structural transformations of the metal particles occurred during the oxidative regeneration process. Temperature programmed reduction (TPR), photoelectron spectroscopy (XPS) and FTIR of adsorbed CO have been used to identify these modifications.