The determination of the local structure of cobalt- or nickel-promoted MoS2-based hydrodesulfurization catalysts is of interest for understanding the mechanism leading to an increased activity brought by cobalt or nickel, the so-called synergetic effect. For that reason, we carried out ab initio calculations using density functional theory under the generalized gradient approximation for periodic systems. The edge substitution model emerges as the most stable structure and provides an excellent agreement with local structures experimentally determined on real catalysts by in situ extended X-ray absorption fine structure. We studied the adsorption of sulfur on the active edge surface of the promoted MoS2 catalyst and determined the equilibrium coverage under sulfiding conditions. It is demonstrated that the incorporation of promoter atoms has a strong influence on the sulfur-metal bond energy at the surface and in particular leads to a reduction of the equilibrium S coverage of the active metal sites. A comparative study on the effects of Co, Ni, and Cu atoms as promoters was performed. Detailed results on the surface electronic structure of promoted MoS2 are presented. (C) 2000 Academic Press.