The hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied over sulfided NiMo/gamma-Al2O3, CoMo/gamma-Al2O3, and Mo/gamma-Al2O3 catalysts. The Ni and Co promoters strongly enhanced the activity of the Mo catalyst in the direct desulfurization pathway of the HDS of DBT and 4,6-DMDBT and in the final sulfur-removal step in the hydrogenation pathway, while the hydrogenation was moderately promoted. H2S had a negative effect on the HDS of DBT and 4,6-DMDBT, which was strongest for the NiMo catalyst and stronger for the direct desulfurization pathway than for the hydrogenation pathway. Because the direct desulfurization pathway is less important for the HDS of 4,6-DMDBT than the hydrogenation pathway, the conversion of 4,6-DMDBT was less affected by H2S than the conversion of DBT. The sulfur removal via the direct desulfurization pathway and the ultimate sulfur removal in the hydrogenation pathway were affected by H2S to the same extent over all the catalysts. This suggests that the removal of sulfur from tetrahydrodibenzothiophenes takes place by hydrogenolysis, like the direct desulfurization of DBT to biphenyl. The CoMo catalyst performed better than the NiMo catalyst in the final desulfurization via the hydrogenation pathway in the HDS of 4,6-DMDBT at all partial pressures of H2S. (C) 2004 Elsevier Inc. All rights reserved.