Molybdenum-based catalysts as hydrodesulfurization (HDS) catalysts are among the most important industrial catalysts. One of the new approaches characterizing the structure of such catalysts and the behavior of sulfur over the "working" catalyst is a S-35 radioisotope pulse tracer method (RPTM). By tracing the behavior of sulfur in the HDS reaction and sulfur exchange reaction over the sulfided Mo/Al2O3 and Co-Mo/Al2O3 catalysts under practical reaction conditions, it becomes possible for us to observe the catalytic behavior of the catalyst in situ. The formation process of active sites. the important role that H2S plays in this process, and the promotion by cobalt for Mo/Al2O3 catalyst have become better understood.