A series of bimetallic cobalt(Co) and molybdenum(Mo) catalysts supported on activated carbon or alumina were prepared, and their activities in thiophene hydrodesulfurization and ethylene hydrogenation were compared. The nitric oxide (NO) chemisorption and temperature programmed desorption(TPD) were used to characterize the nature of the synergistic active sites. Carbon support-ed catalysts also demonstrated the synergism in activity, which is well known for alumina supported catalysts. The specific activity and the adsorption stoichiometry of each site were interpreted from NO chemisorption. The possibility of NO as a probe molecule for sulfide catalysts was provided by the present work.