Tungsten-based hydrotreating catalysts differ from molybdenum-based catalysts in many aspects. Although theoretical studies have offered many insights into the structures and properties of MoS2-based catalysts, no similar study has been reported for WS2-based catalysts. Theoretical studies on tungsten sulfide will provide an increased understanding of the difference between theses two groups of catalysts, and contribute to the development of highly active and selective hydrotreating catalysts. The present study investigates the details of the edge surfaces of unpromoted and Ni(Co)-promoted WS2 catalysts using density-functional theory (DFT) under generalized gradient approximation (GGA) considering the effect of reaction conditions. For unpromoted WS2 catalysts, the edge surfaces favor sulfur coverage of 50% under reaction conditions for both the metal edge and the sulfur edge. Nickel tends to substitute the tungsten on the W edge in the Ni-promoted catalysts, while cobalt prefers to take the position of tungsten at the S edge in Co-promoted catalysts. The incorporated edge structures are stable for both nickel and cobalt relative to individual promoter sulfides. (C) 2004 Elsevier Inc. All rights reserved.