We report density-functional calculations of dibenzothiophene and dimethyldibenzothiophene over different molybdenum sulfide surfaces representing the active sites of the simplest hydrodesulfurization catalysts. Using the adsorption energies and geometries of the molecules on the different sites, we propose an interpretation of the catalytic activities and selectivities published in the literature, which demonstrated the existence of two parallel reaction mechanisms. Among the various possible configurations, eta(1) (S) adsorption on the sulfur edge of the active phase seems to be at the origin of the direct desulfurization of the molecules whereas benzene ring adsorption on the molybdenum edge is at the origin of the hydrogenation pathway. Although a combination of aromatic and steric effects strongly inhibits its adsorption, we show that the presence of stacking defects on the molybdenum sulfides would allow the adsorption and the activation of DMDBT. (C) 2004 Elsevier Inc. All rights reserved.