A series of fluorinated NiMo/Al2O3 catalysts containing different amounts of fluorine were prepared and their activity with respect to the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was compared with that of fluorinated CoMo/Al2O3 catalysts. Fluorine modifies two properties of NiMo/Al2O3 catalysts: metal dispersion and catalyst acidity. In the HDS of DBT, the catalytic activity is enhanced up to 0.5 wt.% added fluorine due to increased dispersion of the metal, and lowered by further fluorine addition because the catalysts lose a significant amount of initial surface area. The hydrogenation of the DBT aromatic ring is enhanced to a greater extent than the hydrogenolysis of the C-S bond due to the fluorine addition. In the HDS of 4,6-DMDBT, however, the catalytic activity is enhanced in proportion to the fluorine content up to 5.0 wt.% added fluorine. The hydrogenolysis of the C-S bond is enhanced to a greater extent than the hydrogenation of aromatic rings, in contrast to the trend observed in the HDS of DBT. A facilitated migration of methyl groups in the aromatic ring of 4,6-DMDBT due to an increase in the amounts of acidic sites of the catalysts is responsible for the enhanced hydrogenolysis of the C-S bond. The optimum fluorine content to yield the maximum amounts of either the direct desulfurization (DDS) or ring-hydrogenated (HYD) products is different depending on the reactants, DBT or 4,6-DMDBT, as well as the catalysts, NiMo/Al2O3 or CoMo/Al2O3. The characteristic reaction results obtained in this study can be explained by considering the relative contributions of the rates of ring hydrogenation and methyl-group migration to product distribution for different cases of reactants and fluorinated catalysts. (C) 2003 Elsevier Science B.V. All rights reserved.