Silica-supported cobalt phosphide (CoxPy/SiO2) catalysts, prepared from precursors having P/Co molar ratios in the range 0.50-2.0, were characterized and evaluated for thiophene hydrodesulfurization (HIDS). The HDS activities of the COxPy/SiO2 catalysts varied nearly four-fold depending on the composition of the Co phosphide phase, with the most active COxPy/SiO2 catalyst corresponding to the precursor composition having just enough P to ensure formation of phase-pure CoP on the silica support. The maximum in HIDS activity observed for this Co phosphide composition can be correlated with a balance between site density (as measured by 02 chemisorption) and resistance to S incorporation (as measured by S analysis of catalysts subjected to an H2S/H-2 treatment). (C) 2008 Published by Elsevier B.V.