Silica-supported molybdenum phosphide, MoP/SiO2, and tungsten phosphide, WP/SiO2, were prepared and characterized for their catalytic activity in hydrodenitrogenation (HDN) and hydrodesulfurization (HDS). The silica-supported phosphides were tested in the simultaneous hydroprocessing of quinoline and dibenzothiophene at 3.1 MPa and 643 K and were compared to a MoS2/SiO2 sample. The supported phosphides had superior HDN and lower HDS activity compared to the sulfide under these conditions. The conversion levels Of MOP/SiO2 were similar to those of bulk MoP, while the WP/SiO2 was somewhat less active than its bulk WP counterpart. However, when compared on a CO uptake basis, the bulk materials had a clear performance advantage. Solid state nuclear magnetic resonance measurements indicated P-31 shifts from 85% H3PO4 of 255 ppm for WP and 214 ppm for MoP. Comparison of freshly prepared and passivated samples indicated that the supported phosphides were susceptible to surface oxidation on exposure to the atmosphere as a result of their small particle size.