Sulfided Ni-Mo catalysts prepared by homogeneous sulfide precipitation (HSP) and impregnated thiosalt decomposition (ITD) methods were used in thiophene HDS, pyridine HDN, and simultaneous HDS/HDN reactions. All catalysts were characterized by BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, and X-ray photoelectron spectroscopy. The effect of various reaction parameters such as catalyst composition, method of catalyst synthesis, reaction temperature, and reaction pressure on catalytic behavior has been investigated. The inhibition/enhancement effects in simultaneous HDS/HDN reactions were also examined. The studies conducted over the unsupported catalysts present trends very similar to those obtained over the supported catalysts and suggest the intrinsic nature of active sites to be essentially independent of the support.