Effects of the H2S partial pressure on the catalytic activity and product selectivity of the hydrodesulfurization (HDS) reactions of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) were investigated over the sulfided NiMo/Al2O3 and NiW/Al2O3 catalysts under the deep desulfurization conditions (sulfur concentrations <0.05 wt.%). The inhibiting effects of H2S were observed in the HDS reactions of both DBT and 4,6-DMDBT. The HDS reaction of DBT was more strongly inhibited by H2S than that of 4,6-DMDBT over each catalyst. The formations of both biphenyls (BPs) and cyclohexylbenzenes (CHBs) were inhibited by H2S, but the former were inhibited more strongly. The NiMo/Al2O3 catalyst was less susceptible to the inhibiting effect of H2S than the NiW/Al2O3 catalyst. The promotion of Ni on NiMo catalyst reduced the inhibiting effect of H2S on the HDS activity. The HDS reactions were expressed by the Langmuir-Hinshelwood rate equation to examine the mechanisms of the inhibiting effects of H2S over the various catalysts. Over each catalyst, the heats of adsorption increased in the order DBT < 4,6-DMDBT < H2S. Since H2S was adsorbed on the catalyst more strongly than DBTs, the HDS reactions of DBTs were inhibited.