A series of W/Al2O3 catalysts containing various amounts of phosphorus (0-6 wt.-% P2O5) were characterized, in the oxide state by X-ray diffraction and temperature-programmed reduction (TPR), and in the sulphide form by high-resolution transmission electron microscopy (HRTEM); their activities for simultaneous hydrodesulphurization (HDS) of gas oil and hydrodenitrogenation (HDN) were also evaluated. The TPR results indicated that phosphorus addition increased the amount of those tungsten species which are more easily reduced, such as octahedral polytungstates. Consistently, the HRTEM micrographs showed that when phosphorus content was increased the lateral dimension of the WS2 structures did not significantly vary, while the number of stacks was clearly increased. Both HDS and HDN were notably enhanced by increasing the phosphorus content; the promotion being relatively larger for HDS than for HDN. Possible explanations for the phosphorus promotion on W/Al2O3 catalysts are discussed.