To investigate the behavior of sulfur during the hydrodesulfurization (HDS), S-35-labeled dibenzothiophene ([S-35]DBT) was reacted on a sulfided Mo/Al2O3. It was found that S-35 in [S-35]DBT was accommodated on the catalyst and the concentration of S-35 On the catalyst always reached a steady state under fixed reaction conditions. S-35 accommodated os the catalyst cannot be removed without the incorporation of sulfur from HDS of sulfur compounds such as DBT, benzothiophene, thiophene, and thiophenol. The removal rate of S-35 from the catalyst depended upon the rate of HDS of these compounds, that is, the amount of sulfur incorporated into the catalyst. It was suggested that H2S is formed from some portion of sulfur on the surface of the catalyst other than from that in the sulfur compounds. When hydrodenitrogenation (HDN) and hydrodeoxygenation (HDO) reactions were carried out on the-catalyst containing S-35, some portion of S-35 could be replaced by oxygen atoms and released as [S-35]H2S; in contrast to this, S-35 was hardly replaced by N atoms.