Ordered mesoporous materials (SBA-15 and KIT-6 silica and MFI zeolite) supported nickel phosphide (NixPy) hydrotreating catalysts were prepared by reduction of oxidic precursors with an initial stoichiometric Ni/P ratio of 2 The metal loading (20 and 30 wt.% NixPy) and pretreatment conditions (773 K or 873 K reduction temperature, in situ sulfidation at 723 K) of the precursors were varied. Temperature programmed reduction, in situ XRD, and 31P NMR indicate the formation of metallic nickel then different nickel phosphides (Ni3P, Ni12P5, then Ni2P) in this order upon reduction. The changes in the textural properties of the catalysts compared to their parent supports promote the conclusion that a significant part of the NixPy phases is located inside the mesopores. The catalytic activity (parallel dibenzothiophene hydrodesulfurization and o-methyl aniline hydrodenitrogenation) increases strongly with increasing NixPy loading. The KIT-6 and SBA-15 supported catalysts exhibit higher hydrotreating activities than reference CoMo/Al2O3 and Ni12P5/SiO2 catalysts. In contrast, the catalyst based on a mesoporous MFI support had the lowest hydrotreating activity. This activity trend is explained by the propensity of high-surface area mesoporous silica supports to well disperse metal phosphide particles. The active phase composition of the spent catalysts is in the range of Ni2.0–2.6P1.0S0.4–0.7. This suggests that bulk Ni2P with some sulfur in its surface forms the active phase in the mesopores of SBA-15 and KIT-6.