The effect of the cobalt addition on the H2S inhibition and subsequently on the polychlorinated biphenyl (PCB) hydrodechlorination (HDC) activity over Mo-based catalysts was investigated. The HDC activity over a Mo catalyst containing cobalt was much higher than that over the same Mo catalyst without cobalt. On the other hand, the HDC activity of the Co–Mo catalyst was more inhibited by H2S than that of the Mo catalyst at 300 C. Thus, kinetic parameters were calculated using a Langmuir–Hinshelwood model to determine the reaction pathway of the H2S inhibition over Mo and Co–Mo catalysts. We found that the heats of adsorption of PCB (Aroclor 1242) and H2S on the Co–Mo catalyst was higher than on the Mo catalyst, indicating that the sulfur-containing species adsorb more strongly on the catalyst containing cobalt. The results suggested that while the Co–Mo catalyst was more inhibited by H2S, the Mo–S bonds were more stable on this catalyst than on the solely Mo catalyst. This Mo–S bond was responsible for the stabilization of the active phase, which allowed creation of a greater amount of sulfur atoms potentially labile. Thus, that explained the better HDC activity over the Co–Mo catalyst than over the Mo catalyst, despite a greater H2S inhibition on the former.FONACYT G-2005000444 PDVSA 2005-00051/2005-024