The hydrodenitrogenation (HDN) of ortho-propylaniline (OPA) has been studied over NiMoP/Al2O3 catalysts with 0-4 wt% phosphorus, 0-3 wt% nickel and 0-12 wt% molybdenum. The HDN reaction was found to proceed via two reaction paths : hydrogenation of the phenyl ring followed by NH3 elimination to propylcyclohexene, which further (de)hydrogenated to propylcyclohexane and propylbenzene, and direct C(sp(2))-N bond hydrogenolysis to propylbenzene. The first denitrogenation path was promoted by Ni and inhibited by the presence of H2S, where hydro enation of the phenyl ring was suggested to be the rate-limiting step. The second denitrogenation path was promoted by phosphorus and inhibited by the presence of H2S and Ni. It is suggested that at least two different kinds of catalytic sites are acting in the HDN of OPA over a sulphided Mo-based catalyst. The first site bears the characteristics of the Ni-Mo-S phase, is strongly promoted by Ni and is responsible for the hydrogenation reaction. The second site is a site associated with Mo, its activity is enhanced by phosphorus, but diminished by Ni and H2S, and it is specially active for the C(sp(2))-N bond cleavage reaction. Both sites are different from the catalytic site which is responsible for the aliphatic C(sp(3))-N bond cleavage.