The inhibitive effect of nitrogen-containing compounds on hydrodenitrogenation was studied over NiMo(P)/Al2O3 catalysts. From the differences in the adsorption constants it is concluded that at least four distinct catalytic sites are involved in the elementary hydrodenitrogenation steps. The catalytic site for the cleavage of aliphatic C(sp(3))-N bonds is acidic and most probably an SH species on a surface Mo or Ni site. The catalytic site for the hydrogenation of a phenyl group is different from that for alkene hydrogenation, the former being more coordinately unsaturated than the latter, with two or three sulphur vacancies. A fourth site is responsible for the hydrogenolysis of the C(sp(2))-N bond of anilines. It is characterised by a more reduced environment of Mo and by phosphorus promotion.