The surface chemistry of CO on reduced and sulfided Ni2P/SiO2 catalysts has been investigated with infrared (IR) spectroscopy. There are four different v(CO) absorbances observed for adsorbed CO, on reduced and sulfided Ni2P/SiO2 catalysts with the following assignments: (1) CO terminally bonded to cus NOdelta+ (0 < delta < 1) sites (2083-2093 cm(-1)); (2) CO adsorbed on Nidelta+ (0 < delta < 1) bridge sites (1914 cm(-1)); (3) CO terminally bonded to P (similar to2200 cm(-1)); and (4) formation of Ni(CO)(4) (2056 cm(-1)). At 298 K, CO terminally bonded to cus Nidelta+ (0 < delta < 1) sites on reduced Ni2P/SiO2 catalysts exhibited a characteristic vco absorbance at 2083 cm(-1), while treatment of Ni2P/SiO2 catalysts with a 3.03 mol % H2S/H-2 mixture shifts this vco absorbance to 2093 cm(-1). The enthalpy of adsorption (DeltaH(ads)) of CO on the Ni2P/SiO2 catalysts was estimated by using a van't Hoff plot: DeltaH(ads) = -31.4 +/- 1.5 and -30.5 +/- 1.5 kJ/mol for reduced and sulfided Ni2P/SiO2 catalysts, respectively. The IR spectroscopic data for the adsorption of CO on reduced and sulfided Ni2P/SiO2 and Ni/SiO2 catalysts correlate well with the trend of hydrodesulfurization (HDS) activity of catalysts. Specifically, we observe a higher site density and higher DeltaH(ads) for CO adsorption on the Ni2P/SiO2 catalysts compared to both reduced and sulfided Ni/SiO2 catalysts. The IR spectroscopic and HDS catalytic results are consistent with the formation of a surface phosphosulfide phase (i.e. NiPxSy) on the Ni2P/SiO2 catalysts under sulfiding conditions.