The influence of vanadium additives on the activity of nickel-molybdenum-alumina (NMA) catalysts in the hydrotreating of petroleum feedstock was studied. A possibility of enhancing the hydrogenating and hydrodesulfurizing activity of these catalysts by introducing this additive was shown. A model explaining the modifying effect of the additive was proposed. It was assumed that molybdenum 12-heteropoly compounds were formed on the NMA catalyst surface during synthesis.