A commercial Co-Mo/Al2O3 catalyst was labeled with the radioisotope S-35 in hydrodesulfurization (HDS) of S-35-labeled dibenzothiophene (S-35-DBT) in a high-pressure flow reactor at 50 kg/cm(2). Then, HDS of 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) or sulfur exchange of H2S were carried out on the labeled catalyst at 50 kg/cm(2) and 260-360 degrees C. The amounts of labile sulfur participating in the reaction were determined from the radioactivity of S-35-H2S released from the S-35-labeled catalyst. In the HDS reactions, the amount of labile sulfur participating in the reaction decreased in the order : DBT > 4-MDBT > 4,6-DMDBT. In the sulfur exchange reaction with H2S, the adsorption of H2S on the catalyst reached saturation above a H2S partial pressure of 0.36 kg/cm(2). It was suggested that the release of H2S from the labile sulfur may be the rate determining step of the HDS reaction.