The formation and stable structures of surface hydrogen species on unpromoted and nickel-promoted MoS2 catalyst surfaces is studied by investigating the adsorption and dissociation of molecular hydrogen and hydrogen sulfide using periodic DFT calculations under generalized gradient approximation (GGA). On the stable MoS2 (1 0 (1) over bar 0) Mo-edge, the dissociation of hydrogen with the formation of -SH groups is an exothermic process and energetically most favoured. On a partially nickel-promoted (1 0 1 0) surface, the dissociation of molecular hydrogen to form a surface -SH group and a Mo-H or Ni-H species is an endothermic process, and requires higher energy than on an unpromoted (1 0 (1) over bar 0) Mo-edge surface. On a fully promoted (1 0 1 0) Ni-edge, associative adsorption of H-2 is exothermic, and dissociation to a pair of Ni-H and -SH groups or two Ni-H species is endothermic. Hydrogen sulfide can be strongly adsorbed on the vacant nickel sites on the partially and fully nickel-promoted (1 0 (1) over bar 0) Mo-edge surface, and surface -SH groups can be formed by dissociation of adsorbed H2S. (c) 2005 Elsevier B.V. All rights reserved.