A S-35 radioisotope pulse tracer method was used to elucidate the behavior of sulfur on a sulfided Co-Mo/Al2O3 under the practical hydrodesulfurization (HDS) conditions. S-35-labeled H2S (S-35-H2S) and dibenzothiophene (S-35-DBT) were synthesized, Then, the exchange reactions with S-35-H2S and the HDS reaction of S-35-DBT were carried out at 200-400 degrees C and 10-50 kg/cm(2), By tracing the change in radioactivity of released S-35-H2S, amounts of labile sulfur involved in the reactions and the rate constants of H2S formation from the labile sulfur were determined. The amount of labile sulfur increased with increasing temperature, and the release rate of S-35-H2S increased with increasing the supply rate of sulfur, The sulfur exchange with H2S was very rapid at each temperature. Comparison of the amounts of labile sulfur and the rate constants of sulfur exchange in HDS reactions with those in sulfur exchange with H2S suggests that the transformation between the labile sulfur and the vacancies on the catalyst surface would predominantly proceed through the sulfur exchange with H2S even in the HDS reactions. It was also found that the addition of H2S inhibited the HDS reaction of DBT while it increased the sulfur exchange rate.