2-Methylpyridine and 2-methylpiperidine strongly inhibited the hydrogenation pathway in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene over sulfided NiMo/gamma-Al2O3, CoMo/gamma-Al2O3, and Mo/gamma-Al2O3 catalysts at 340 degrees C, 4.8 MPa H-2, and 35 kPa H2S. The direct desulfurization (DDS) pathway was inhibited by both amines over the Mo and CoMo catalysts as well, but the NiMo catalyst showed a promotion effect in the HDS of DBT at 300 degrees C and at low partial amine pressure. Experiments with other amines showed that only the more basic and stable heterocyclic amines promoted the DDS pathway of DBT over NiMo. This promoting effect of amines on NiMo is explained by the special location of the Ni promoter atoms, on the metal edge of the MoS2 particles with low sulfur coverage. The perpendicular adsorption of amines hinders especially the pi adsorption of DBT and 4,6-dimethyldibenzothiophene and thus the hydrogenation pathway, whereas the filling of sulfur vacancies by H2S inhibits the DDS pathway. (c) 2006 Elsevier Inc. All rights reserved.