The poisoning effect of carbazole and quinoline on the performance of sulfided CoMo/Al2O3 in the hydrodesulfurization (HDS) of dibenzothiophene (DBT), 4-methyidibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) has been examined. The HDS of 4-MDBT or 4,6-DMDBT is retarded by the presence of the nitrogen compounds in the reaction stream even at low concentrations, e.g. 50 ppm, while the HDS of DBT remains nearly unaffected by these compounds unless the concentrations are high, in excess of 500 ppm. The nitrogen compounds suppress the ring-hydrogenation step (HYD) more than the direct desulfurization step (DDS) in the HDS of DBT, while an opposite trend is observed in the HDS of 4-MDBT or 4,6-DMDBT, i.e. DDS is suppressed to a greater extent than HYD. The nitrogen compounds poison acidic sites on the catalyst, as confirmed by the reduced amounts of pyridine adsorbed and by the suppressed isomerization of 2,2 ' -dimethylbiphenyl on the poisoned catalyst. Quinoline exhibits a stronger poisoning effect than carbazole. The HDS of the three DBT compounds studied in this work are suppressed to different extents by the nitrogen compounds because the extent of steric hindrance to the C-S-C bond by methyl groups attached to the ring structure is different depending on the DBT compounds.