The effect of phosphorus in the hydrodenitrogenation (HDN) of piperidine and pyridine over NiMo/A1203 catalysts was studied. In the absence of H2S no effect of P was found, but in the presence of H2S the HDN activiiy of NiMo/Al203 catalysts decreased as the P content In the catalysts increased. It is concluded that phosphorus has a negative effect on the C-N bond cleavage in piperidlne, as well as on the subsequent hydrogenation of the Cs alkene intermediates. Also the influence of Ni was found to depend on the reaction conditions. A substantial promotion effect of NI could only be observed In the presence of H2S, both in the C-N bond cleavage of piperidine and in the pyrMine hydrogenation. The activity for the HDJ of piperidlne of NiMo(P)/Al203 catalysts increased with increasing H2S concentration up to H2S/H2 = 12.5~10 mdlmol.