In hydrodesulfurization (HDS) of S-35-labeled dibenzothiophene ([S-35]DBT) performed on the catalyst derived from alumina-supported ruthenium carbonyls-cesium hydroxide systems, the effect of the amount of ruthenium loaded on the catalytic activity, the amount of labile sulfur and the rate constant of H2S release were elucidated by tracing the behavior of S-35 on the working ruthenium catalysts. The HDS of S-35-DBT was performed in conditions: 300 degrees C, 50 kg/cm(2), WHSV 28 h(-1), H-2 25 l/h, DBT 1 wt%, precursor Ru-3(CO)(12)-3CsOH/Al2O3 (Ru 0-20 wt%). During the reaction with sulfided catalysts, the changes in the radioactivities of unreacted [S-35]DBT and product [S-35]H2S With reaction time were monitored. The conversion of DBT increased with increasing the amount of ruthenium loaded up to 16 wt% approximately linearly, and slightly decreased with the further addition of ruthenium. This indicates that the high dispersion of ruthenium species could be obtained and was kept up to 16 wt%. The amount of labile sulfur on the catalyst (S-0), which was calculated from the maximum amount of S-35 accommodated On the catalyst, increased with increase in the amount of ruthenium loaded and reached the maximum at Ru=16 wt%, which leveled off with further addition of ruthenium. These results show that the active sites on the catalyst increased with increasing the amount of ruthenium loaded up to 16 wt%. The rate constant of [35S]H2S release (k(re)) was estimated from the first order plots of the increasing or decreasing radioactivities of product [S-35]H2S. The values of k(re) slightly decreased with increase in the amount of ruthenium loaded, indicating that the mobility of sulfur on the catalysts decreased with increasing ruthenium. This would be due to the increase in the amount of cesium loaded with increasing the amount of ruthenium loaded.