The effect of H2S on hydrodenitrogenation (HDN) of pyridine over alumina supported Mo, Ir and mixed Ir-Mo sulfide catalysts was studied. The pyridine hydrogenation was inhibited by H2S on all catalysts, while the cleavage of the C-N bond of intermediate piperidine was facilitated on the Mo and inhibited on the Ir and Ir-Mo catalysts. The promotional effect between Ir and Mo was observed in the piperidine cleavage both in the presence and absence of H2S.