The sulfidation state in a series of Co-promoted Mo/Al2O3 catalysts was investigated using a S-35 pulse tracer method. S-35-labeled H2S ([S-35]H2S) pulses were introduced into catalysts in a nitrogen stream until the radioactivity in the recovered pulse approached the radioactivity of the introduced pulse. From the amount of introduced [S-35]H2S, the amount of sulfur accumulated on the catalyst was estimated. The result indicated that the amounts of sulfur accumulated on the catalysts increased with increasing temperature for all catalysts. Only molybdenum was sulfided in both Co-Mo/Al2O3 and Mo/Al2O3 catalysts below 300 degrees C, but the sulfided states of the catalysts at 400 degrees C were very close to the stoichiometric states where Co and Mo are present as Co9S8 and MoS2. Further, hydrodesulfurization (HDS) reactions of radioactive S-35 labeled dibenzothiophene were carried out over the series of Co-promoted Mo/Al2O3 catalysts. The amount of labile sulfur and the release rate constant of H2S were determined. The promotion effect of cobalt on activity of the molybdenum catalyst was attributed to the formation of more active sites. Moreover, the increase in the catalytic activity with Co/Mo ratio among the promoted catalysts was due to increase in the number of the sites with the same activity.