Catalysts consisting of Ni2P placed on low and high surface area siliceous supports, i.e. SiO2-L, SiO2-H, and Si-MCM-41, were synthesized by TPR (temperature-programmed reduction), and the effect of the sulfur and nitrogen content of the feed on hydrotreating activity was studied. Structural information on the supported Ni2P phase after reaction was obtained by XRD (X-ray diffraction) and EXAFS (extended X-ray absorption fine structure) measurements. The catalytic activity in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) was obtained at 573 and 613 K and a pressure of 3.1 MPa in an upflow, liquid-gas-solid bed reactor with a feed consisting of 4,6-dimethyldibenzothiophene (4,6-DMDBT) and quinoline dissolved in tridecane as solvent. Using reference compositions of 4,6-DMDBT at levels of 500 ppm S, dimethyldisulfide (DMDS) at levels of 6000 ppm S, and quinoline at levels of 200 ppm N, the order of activity was Ni2P/SiO2-L < Ni2P/SiO2-H < Ni2P/MCM-41, compared on the basis of equal number of sites (230 mmol) placed in the reactor. The Ni2P/MCM-41 gave a notably high conversion in HDS of 98% that was substantially above those of a Ni-Mo-S/Al2O3 commercial catalyst which had an HDS conversion of 81%, on a basis of equal quantity of sites (230 mmol) placed in the reactor. The sites were titrated by pulse CO uptakes for the phosphides and by pulse O-2 adsorption at low temperature for the sulfide. Analysis of the spent samples by EXAFS showed that Ni(2) sites of square pyramidal geometry in Ni2P are bound to sulfur, with a lower Ni-S coordination, as the particle size decreased, and the order in the number of these sites followed the reactivity Ni2P/SiO2-L < Ni2P/SiO2-H < Ni2P/MCM-41. It is thus concluded that the active Ni(2) site on the highly dispersed Ni2P is much more tolerant to sulfur than the tetrahedral Ni(1) sites also present in the samples.