Al2O3-supported Mo, Ni, Ni---Mo, and Rh catalysts, prepared by sequential aqueous impregnation and in situ sulfidation, were investigated in the hydrodenitrogenation (HDN) of quinoline at 643 K and 3 MPa and in the hydrodesulfurisation (HDS) of thiophene at 673 K and 0.1 MPa. The Ni and Mo catalysts had a very low conversion of quinoline to hydrocarbons which improved only slightly in the presence of phosphate. The Rh catalysts had a high conversion and a high selectivity for propylcyclohexane and showed no deactivation with time. The addition of Ni to MO/Al2O3 and of phosphate to Ni---Mo/Al2O3 and Rh/Al2O3 catalysts increased the HDN conversion significantly. The selectivity for propylbenzene and the apparent HDN activation energy increased with increasing P-loading. Ni increased the thiophene conversion of Mo/Al2O3, but phosphate had almost no influence on the HDS activity of Ni---Mo/Al2O3 and Rh/Al2O3. The effect of phosphate is due to a combination of structural and catalytic factors. Phosphate improves the activity by inducing the formation of the type II Ni---Mo---S structure, but also, especially at high Ni loading, lowers the activity by inducing a decrease in the dispersion of the Ni---Mo---S phases and a segregation of Ni3S2. Phosphate also promotes the S- and N-elimination reactions, but this only has an influence on the overall catalyst activity if the preceding hydrogenation reactions are not rate determining.