A standard Ni-Mo/gamma-Al2O3 catalyst containing 4 wt% Ni was modified by addition of nickel (2 wt%) and, alternatively, of vanadium (4 wt%) by contacting with a solution of the respective metal naphthenate. The catalysts were sulfided and tested in a batch reactor at 350-degrees-C and 165 bar for hydrogenation of naphthalene and for hydroprocessing of dibenzothiophene. Reaction networks were determined for each reactant, and the dependence of the pseudo first-order rate constants on the amount of nickel and of vanadium in the catalyst was used to determine the effects of nickel sulfide and of vanadium sulfide deposits on catalyst performance. For example, the nickel sulfide deposits only slightly affected the rate constants for hydrogenation in either network, but the vanadium sulfide deposits led to a decrease of at most 50% in the rate constants for hydrogenation reactions in the naphthalene network and to a doubling of the rate constants for hydrogenation reactions in the dibenzothiophene network. The nickel sulfide deposits led to almost no change in the rate constant for hydrogenolysis of dibenzothiophene (to give biphenyl), but the vanadium sulfide deposits led to a threefold decrease in the rate constant for this reaction. The nickel sulfide deposits have little activity for reactions giving lower-molecular-weight ("cracking") products, but the vanadium sulfide deposits have a relatively high activity for cracking, which suggests that they are acidic. The effects of the deposits are complex, as they both block catalytic sites and form new ones. The results indicate a need for representing the nickel and sulfide deposits separately in process models for heavy oil hydroprocessing.