The hydrodesulfurization of dibenzothiophene on a commercial CoMo/Al2O3 catalyst was studied in a multiphase reactor. The operating conditions were varied over the following range : temperatures, 513-573 K; total pressures, 50-80 bar; molar hydrogen to hydrocarbon ratios, 1.1-4.1. Hougen-Watson rate equations for the hydrogenolysis of dibenzothiophene into biphenyl and H2S, for the hydrogenation of dibenzothiophene into tetra- and hexahydrodibenzothiophene, for the hydrogenation of biphenyl into cyclohexylbenzene, and for the subsequent hydrogenation of cyclohexylbenzene into bicyclohexyl were developed. Two different types of active sites were considered : sigma sites for hydrogenolysis and tau sites for hydrogenation. The surface reaction between adsorbed reactants and two competitively adsorbed hydrogen atoms was found to be the rate-determining step for both types of reaction.