This dissertation describes preparation and characterization of Ni₂P catalysts and their application in deep hydrodesulfurization (HDS) of a model sulfur compound, 4,6-dimethyldibenzothiophene (4,6-DMDBT), one of the most refractory S-compounds. This topic is of great importance in addressing recently enacted environmental regulations limiting the sulfur content in fuels. The work carried out includes synthesis of Ni₂P on different siliceous supports, SiO₂, MCM-41, and ultra-stable Y zeolite (USY). It also includes determining the characteristics of the supported Ni₂P catalysts with a wide range of techniques: X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) and X-ray absorption fine structure (XAFS) spectroscopy. The use of these techniques allowed better understanding of the nature of the active sites as well as the effect of supports. Activity tests were conducted in the HDS of 4,6-DMDBT and the HDN of quinoline. The performance of the catalysts will be compared to that of a conventional sulfide hydrotreating catalyst, Ni-Mo-S/Al₂O₃. Investigation of the reaction mechanism in the hydrodenitrogenation (HDN) of 2-methylpiperidine together with in situ FT-IR measurements were conducted to understand how catalyst properties affect activity and selectivity.