The radioisotope tracer method has been used to quantify the behavior of sulfur on sulfided Ni-Mo/Al2O3, Mo/Al2O3, and Ni/Al2O3. The apparent activation energies of HDS reaction for DBT for the three catalysts were 20 +/- 1 kcal/mol. The formation rate constants of S-35-H2S were determined and the amount of labile sulfur on the sulfided catalysts was estimated by tracing the changes in radioactivities of the unreacted S-35-DBT and the formed S-35-H2S during the HDS reaction of S-35-labeled dibenzothiophene (S-35-DBT). It was deduced that ca. 75% of sulfur in the sulfided Mo/Al2O3 was related to HDS reaction at infinite rate of HDS. Compared with the amounts of labile sulfur in the sulfided Ni-Mo/Al2O3, Mo/Al2O3, and Ni/Al2O3, it was determined that the amounts of labile sulfur were 1.6, 9.8, and 18.4 mg sulfur/g catalyst at 280 degrees C for the three catalysts, respectively. And it was suggested that the sulfur in the form of NiS on the sulfided Ni-Mo/Al2O3 was not labile and that the sulfurs attached to both Mo and Ni atom were more labile and related to HDS. The promotion of Ni for Mo-based catalysts was attributed to the sulfur bonded to both Mo and Ni in the MoS2 phase being more labile.