Highly dispersed molybdenum sulfides supported on alumina were found to be prepared by using Mo(CO)(6) as a starting material. The structure and catalytic activities for thiophene hydrodesulfurization (HDS) and butadiene hydrogenation (HYD) of the molybdenum sulfides and the effects of modification with Co by using Co-2(CO)(8) or Co(NO)(CO)(3) as a precursor were examined as a function of the dispersion of the molybdenum sulfide species. The structure and dispersion of molybdenum sulfides were studied by EXAFS techniques. The TOF (activity per Mo atom) of HDS remained invariant at a high dispersion of Mo, whereas the TOF of HYD increased with increasing dispersion of the molybdenum sulfide species. Dinuclear molybdenum sulfide species showed a significantly high TOF for the HYD. It was found with Mo/Al2O3 catalysts promoted with Co that both the promotional ratio in the HDS activity and the amount of Co required for the maximum activity increased as the dispersion of the molybdenum sulfide increased.