Quantum chemical calculations for H 2 dissociation on NiMoS and MoS 2 catalysts were carried out using the ab initio density functional theory and the pseudopotential approaches. The catalysts were modeled by XMo 4S 17H 16, X=Ni, Mo cluster models. Two dissociation route, the heterolytic and the homolytic, were studied on the MoS 2 cluster model. The results show that the homolytic dissociation is favored over the heterolytic route. On the NiMoS cluster model, the homolytic route is less favored thermodynamically than in the MoS 2. The topological analysis of the electron density indicates the presence of S 2 groups on the MoS 2 and NiMoS surfaces, as well as a Mo–Mo bond in the MoS 2 model. No Ni–Mo bond was found in the NiMoS cluster model. The presence of Ni activates the Mo–S bond, produces a polarization at this bond and decreases the chemical hardness of the solid.