The hydrodemetalization (HDM) kinetics of nickel 5,10,15,20-tetraphenylporphyrin (Ni-TPP) has been studied at 613 K in the presence of sulfided Mo/Al2O3 catalysts. The reversible sequential mechanism, in which the intermediate compounds nickel 5,10,15,20-tetraphenylchlorin (Ni-TPC) and nickel 5,10,15,20-tetraphenylisobacteriochlorin (Ni-TPiB) are formed, is in good agreement with earlier findings that the kinetic network is governed by the type of porphyrin and independent of the type of catalyst applied or its chemical state. The current study focuses on the influence of the H-2 and H2S concentrations on the various reaction-steps in the kinetic mechanism. H2S promotes the hydrogenolysis step in the HDM of Ni-TPP. Through evaluation of kinetic models in which porphyrins, H-2, and H2S adsorb on various types of sites it is shown that a two-site model, with small inhibition by H-2 and negligible inhibition by H2S, provides a satisfactory description of the HDM of Ni-TPP over Mo/Al2O3.