The hydrodesulfurization of 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene on a commercial CoMo/Al2O3 catalyst was studied in a multiphase flow reactor in the presence of dibenzothiophene. Hougen-Watson rate equations for the hydrogenolysis of 4-methyldibenzothiophene into 3-methylbiphenyl and H2S and for the hydrogenation of 4-methyldibenzothiophene into partially hydrogenated intermediates were developed. In an identical manner Hougen-Watson kinetics were determined for the hydrogenolysis of 4,6-dimethyldibenzothiophene into 3,3＇-dimethylbiphenyl and H2S and for the hydrogenation of 4,6-dimethyldibenzothiophene into dimethyltetra- and dimethylhexahydrodibenzothiophene. Two different types of active sites exist on the catalyst surface : sigma sites for hydrogenolysis and tau sites for hydrogenation reactions. The surface reaction between adsorbed reactants and two competitively adsorbed hydrogen atoms was the rate-determining step for both types of reaction.