The hydrodesulfurization of dibenzothiophene on a NiMoS cluster consisting of 1 Ni and 18 Mo atoms was investigated by means of density functional theory. The calculations focus on the direct desulfurization pathway where dibenzothiophene reacts with H-2 to biphenyl and H2S. Critical steps with high activation energies are breaking of the two C-S bonds of the DBT molecule (ca. 32 kcal/mol), the activation of H-2 by heterolytic dissociation and the removal of the formed biphenyl molecule from the catalyst surface (17 kcal/mol, respectively). All other reaction steps have activation energies of <= 10 kcal/mol. (C) 2007 Elsevier B.V. All rights reserved.