First principles calculations based on spin-polarized density functional theory and the generalized gradient approximation have been used to study the interaction of atomic and molecular hydrogen and sulfur species with bare and Ni modified MoS2(0001). The calculations employ slab geometries and periodic boundary conditions. Our calculations indicate that H-2 can be adsorbed on this surface only when sulfur defect sites are present. In this case a dissociative chemisorption process with an activation barrier of about 22 kcal/mol has been identified. Adsorption of atomic hydrogen is possible on the nondefective surface with binding energies in the range 8-16 kcal/mol. On the sulfur defective surface, the binding energy of atomic hydrogen increases to 62.2 kcal/mol. Similarly, S atoms can adsorb on the bare surface with adsorption energies in the range 33-42 kcal/mol depending on the surface coverage. For the surface with preadsorbed Ni atoms, these atoms can serve as adsorption centers for both H-x and S-x (x = 1,2) species. Additionally, they can assist dissociation of H-2 molecules. As a result of hydrogenation or sulfiding, the interaction of Ni adatoms with the surface decreases by as much as 50% relative to the case of bare Ni atoms. In turn, this leads to a decrease of the diffusion barriers of Ni-H and Ni-S-x (x = 1,2) complexes relative to the case of bare Ni atoms.