Hydrodesulfurization (HDS) reactions of S-35-radioisotope-labeled dibenzothiophene (DBT) were carried out over a series of gamma-Al2O3-supported noble-metal-containing catalysts at 5.00 MPa and at 260 and 280 degrees C. The amount of sulfur (S-TOTAL) accommodated on the catalyst and the amount of labile sulfur (S-0) participating in the reaction were determined using a direct method, the S-35 radioisotope pulse tracer method, which has recently been developed by the authors. It was observed that both S-TOTAL and S-0 increased linearly with an increase in active metal loading. At the same time, it was found that the sulfided test noble metal catalysts corresponded to a S/Pt (Pd) ratio of 0.25 and that almost all the labile sulfur on these catalysts was mobile in the HDS reaction. Further, the activities of both HDS and hydrogenation reactions over the bimetallic catalyst (Pt-Pd) were higher than those of the monometallic catalysts tested together, whereas the synergetic effects observed on the Pt-Pd catalyst were not as significant as in typical Co-Mo catalyst cases. (C) 2000 Academic Press.