The effects of Ni and Zn on the electronic and chemical properties of MoS2 clusters have been investigated using INDO/S and ab initio SCF calculations. The deposition of Ni or Zn atoms on a series of clusters (Mo5S10, Mo6S12, Mo8S18, Mo9S18, MO12S24, and Mo18S36) that resemble the MoS2(0002)-S and MoS2(10 (1) over bar 0)-Mo surfaces produces a substantial reduction (0.5-2 eV) in the stability of the HOMOs of these systems. In all the cases examined, there was an admetal-->MoS2 charge transfer that increased the negative charge on S and reduced the positive charge on Mo. The electronic and chemical perturbations induced by Ni were much larger than those induced by Zn. Ab initio SCF results for Ni-promoted MoS2(10 (1) over bar 0)-Mo clusters showed a simultaneous increase in the electron density on Mo and in the reactivity of this metal toward H-2 and thiophene. On the (10 (1) over bar 0)-Mo surface, Ni facilitated the dissociation of H-2 by largely increasing the stability of the transition state and products for this reaction. The bonding interactions between MoS2(0002)-S clusters and H-2 or thiophene were weak. Ni and Zn adatoms enhanced the chemical activity of the (0002)-S surface by providing active sites for the dissociation of H-2 and the chemisorption of thiophene. The behavior of molybdenum sulfide catalysts in hydrogenation and hydrodesulfurization processes is discussed in light of these results.