Oxygen chemisorption has been studied on sulfided Mo catalysts supported on SiO2 with differing dispersions as a function of pretreatment. It was found that presulfided samples purged in He at 350 degrees C chemisorb about half as much O-2 as samples reduced first in H-2 at 350 degrees C, followed by He purge at the same temperature. From this study in combination with other studies of chemisorption of probe molecules such as H2S, CO, O-2, and NO on sulfided Mo catalysts, a two-site chemisorption model is proposed. The adsorption sites are intrinsically related to the geometric and electronic character of the MoS2 two-dimensional layer structure. Frontier orbital concepts and Hoffmann’s theory of bonding on surfaces are employed to postulate the selectivity in chemisorption of probe molecules on the two adsorption sites and to correlate quantitatively the chemisorption amount of various probe molecules. The model suggests that Mo-S sites with unsaturated coordination located in S-Mo-S layers are adsorption sites for probe molecules H2S, CO, O-2, and NO, while S sites located in S-S layers only adsorb O-2 and NO.