Abstract Ni oxide-Mo oxide-P oxide-Alumina with wide range of P loading (from 1 to 10 wt% P) were prepared by a sol-gel method to elucidate the role of P on the genesis, structural, and catalytic properties of Ni-Mo based hydrotreating catalysts. Specific surface area of catalysts decreased gradually in proportion to the P loading from ∼500 to 260 m2/g. X-ray powder diffraction revealed that both small and large amounts of P within the alumina framework provoke aggregation of Mo related species. 27Al-NMR indicated that a part of octahedral aluminium sites is highly distorted in the presence of Ni, Mo and P. From 31P-NMR measurements, predominant formation of polymeric P oxospecies and AlPO4 was observed after the calcination step. Thiophene HDS activity gave a maximum at 2 wt% of P due to an increase in Mo dispersion. However, large amount of P has a negative effect on HDS activity due to the formation of bulk MoO3. Butane formation during thiophene HDS decreases with P addition which may indicate the segregation of Ni from MoS2.