The kinetics of the hydrodenitrogenation (HDN) of ortho-propylaniline (OPA) was studied over sulfided NiMo(P)/Al2O3 catalysts. A small amount of propylcyclohexylamine was observed in the reaction product, demonstrating that hydrogenation of the phenyl group of OPA was one of the first and rate-limiting steps in the HDN reaction. A Langmuir-Hinshelwood kinetic model with different sites for the HDN and hydrogenation could successfully fit the reaction network. Some kinetic parameters were obtained by varying the initial reactant partial pressure at constant space time and by simultaneous reaction, while others were determined by varying the space time at constant reactant partial pressure. The HDN rate constants and the adsorption constants of OPA and NH3 were higher over the phosphate-containing catalyst than over the phosphate-free catalyst, while the activation energies as well as the heats of adsorption of OPA did not show any difference between the two catalysts. This demonstrates that the nature of the catalyst sites did not change by the introduction of phosphate. The promotive effect of phosphorus in the HDN of OPA arises from an increased number of active sites for the hydrogenation of the phenyl ring of OPA as well as from a stronger adsorption of OPA due to increased MoS2 stacking and therefore less hindered adsorption. Phosphorus shows a promotive effect on the hydrogenation of pure olefins, but a negative effect when nitrogen-containing compounds are present. This is due to a stronger adsorption of the nitrogen-containing compounds on the P-containing catalyst, and hence to a stronger inhibition.