The hydrodenitrogenation (HDN) of quinolone (Q) and decahydroquinoline (DHQ) over a Ni2P catalyst prepared by the reduction of a conventional phosphate precursor and a surface nickel phosphosulfide phase (Ni2P-S) obtained by the reduction of Ni2P2S6 were studied. A reaction network of the HDN of Q over Ni2P was proposed. Both the hydrogenation and C-N bond cleavage activities of Ni2P were enhanced after the introduction of sulfur. The surface sulfur species of Ni2P-S changed significantly after the HDN of DHQ. The thiolate or thiol species were detected as the major sulfur-containing species in the surface of the spent Ni2P-S catalyst. [GRAPHICS] .