The paper reports results of a study on the performance and properties of NiMo catalysts supported on unmodified NaX and NaY zeolites and the same type of zeolites modified by exchange of Na+ ions into Cr3+ ones. Destruction of the zeolite crystal lattice is found to be much greater in the X-type than in the Y-type zeolites. The zeolites impregnated with Mo6+ cations reveal higher hydrodesulfurization (HDS) activity than those modified only by Cr3+ ions. The introduction of Ni2+ cations into the samples of Mo/NaCrX or Mo/NaCrY almost always results in an enhanced HDS activity.