The sulfidation state in a series of nickel- and cobalt-promoted Mo/Al 2 O 3 catalysts was investigated using 3 5 S radioisotope pulse tracer methods (RPTM). Several pulses of 3 5 S -labeled H 2 S ([ 3 5 S ]H 2 S) were introduced into catalysts in a nitrogen stream until the radioactivity in the recovered pulse approached the radioactivity of the introduced pulse. From the amount of [ 3 5 S ]H 2 S introduced, the amount of sulfur accumulated on the catalyst was estimated. The amounts of sulfur accumulated on the catalysts increased with increasing temperature for all catalysts and the cobalt-promoted catalysts showed features similar to those of the nickel-promoted catalysts. It was found that the sulfided states of the catalysts close to the stoichiometric states, where Ni, Co and Mo are present as NiS, Co 9 S 8 and MoS 2 , was attained at 400 o C for all catalysts. Moreover, it was suggested that only molybdenum was sulfided below 300 o C; the sulfidation of nickel oxide and cobalt oxide in the Ni- or the Co-Mo/Al 2 O 3 catalyst is more difficult at lower temperatures and the sulfidation of nickel and cobalt species occurs only at higher temperatures (above 300 o C).