The hydrodenitrogenation (HDN) of quinoline and its reaction intermediates was studied over NiMo(P)/Al2O3 catalysts. Phosphorus exhibited a promotional effect on the HDN of quinoline and ortho-propylaniline and a negative effect on the HDN of decahydroquinoline and on the hydrogenation of 1-methylcyclohexene in its simultaneous reaction with quinoline. The HDN of quinoline was found to take place via two pathways : via the saturated intermediates decahydroquinoline and propylcyclohexylamine, and via the aromatic intermediates 1,2,3,4-tetrahydroquinoline and orthopropylaniline. Although propylcyclohexane is the main HDN product in both reaction paths, formation of propylbenzene is relatively favored in the latter path. Thus, the relative contributions of the two reaction paths in the quinoline HDN network could be estimated from the HDN product composition. In contrast to previous suggestions, the 1,2,3,4-tetrahydroquinoline-ortho-propylaniline reaction path was found to be nonnegligible : About 40% of the quinoline HDN takes place via this reaction path at 643 K, 3.0 MPa, and P-H2S = 6.5 kPa, and about 35% at 623 K. H2S had an effect opposite that of phosphorus on the HDN of decahydroquinoline and orthopropylaniline. H2S as well as phosphorus had a small positive effect on the HDN of quinoline. This is due to a subtle balance between their opposite effects on the HDN of decahydroquinoline as well as ortho-propylaniline. Because of this, and of the opposite effect of phosphorus as web as H2S on the HDNs of these two molecules, the effect of phosphorus and H2S on the HDN of quinoline is a subtle balance between the two HDN pathways, the catalyst composition, and the reaction conditions.