The hydrodenitrogenation (HDN) of pyridine over activated carbon-supported Mo and NiMo sulfided catalysts was studied. Without H2S in the feed the catalysts showed a high conversion of pyridine and of piperidine to C-5 hydrocarbons at the beginning of reaction, followed by a strong deactivation. A large increase in the conversion of pyridine was observed when H2S was added to the feed at H2S/H-2 greater than or equal to 0.006. The conversion of piperidine to C-5 hydrocarbons was enhanced by H2S and it increased as the H2S pressure increases up to H2S/H-2 greater than or equal to 0.006. The promotion effect of Ni was operative only when H2S was present and it increased with the H2S/H-2 ratio. Without H2S, the degree of surface oxidation of the support affected the deactivation of the catalysts. When H2S was added to the feed, the conversion of pyridine was stable and independent of the acid treatment of the support. A more oxidized support enhanced the C-N bond breaking reaction.