The chemical and electronic properties of a series of Zn/S/Mo(110) and Co/S/Mo(110) systems have been investigated using photoemission, thermal desorption mass spectroscopy, and hydrogen (H-2, D-2, or D) chemisorption, Sulfur multilayers supported on Mo(110) are very reactive toward admetals like Zn and Co. The behavior of the Zn/S/Mo(110) and Co/S/Mo(110) systems indicates that Zn and Co promote Mo <----> S interactions, inducing the formation of molybdenum sulfide films. The ZnMoS and CoMoS films were unreactive toward H-2 or D-2 under ultrahigh vacuum conditions. As gas-phase hydrogen atoms (D) impinged on the surfaces, gaseous hydrogen sulfide was formed. Thus, the slow step in the D-2.gas + S-solid --> D2Sgas reaction is the dissociation of molecular hydrogen. A good correlation exists between trends seen in the hydrodesulfurization (HDS) activity of ZnMoS and CoMoS catalysts and trends found for the sulfidation of Mo and hydrogenation of S in ZnMoS and CoMoS films. The systems that contain Co show the larger HDS activity, the stronger metal <----> metal interactions with a subsequent increase in the reactivity of Mp toward S-containing molecules, and the bigger tendency to create unsaturated Mo sites through the hydrogenation of Mo-bonded S atoms.