Despite the intense research efforts directed toward understanding the microscopic mechanism of hydrodesulfurization reactions on transition-metal-sulfide catalysts, the nature of active sites remains an open question. Industrial catalysts are mostly based on supported highly dispersed MoS2, There is a general agreement that the active centers are coordinatively unsaturated sites at the edge surfaces oriented parallel to the hexagonal axis of this layered sulfide, but the precise local structure is still unknown. In the present ab initio study, we show that the nature and the concentration of the active sites as well as the shape of the MoS2 crystallite can vary with the chemical potentials in the reactive atmosphere above the surface. We also report a precise investigation of the surface structure, chemical composition, and electronic properties of the MoS2 edge surface under working conditions. (C) 2000 Academic Press.