Nickel sulfide catalysts supported on a NaY zeolite with two different metal loadings were prepared by ion exchange and with or without a subsequent NaOH treatment. The catalysts were characterized by number of techniques and measurements, such as X-ray powder diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, energy-dispersive X-ray analysis using a scanning electron microscope, ion back-exchange, and acidity determination. The data showed that after sulfidation, nickel is homogeneously distributed for low Ni loadings, while at high Ni loadings nickel is inhomogeneously distributed and partially located outside the zeolite pores. The NaOH pretreatment enhanced the heterogeneity of Ni distribution and its migration to the external surface of the zeolite, particularly for the high-Ni-containing catalyst. Simultaneously, NaOH pretreatment enhanced significantly the degree of nickel sulfidation. The activities of the catalysts for gas oil hydrodesulfurization (HDS) and pyridine hydrodenitrogenation (HDN) were measured simultaneously at high pressure. The results showed that low-Ni-containing catalysts, and particularly the NaOH-treated one, have the highest intrinsic activity for gas oil HDS. This is due to the combined effect of both their better dispersion and better sulfidation of the Ni phase. For pyridine HDN, the more active catalysts were those with higher acid site density, i.e., the ones not treated with NaOH.