The surface chemistry of thiophene on reduced and sulfided Ni/SiO2 and Ni2P/SiO2 catalysts has been investigated by using infrared (IR) spectroscopy. Thiophene is quite reactive on Ni2P/SiO2 catalysts; even at 190 K, cleavage of some C-S bonds was observed, forming 1,3-butadiene-like species on the surface. Annealing this thiophene-dosed Ni2P/SiO2 catalyst to 250 K resulted in the formation of adsorbed butenes and other thiophene decomposition products on the surface. In the presence of H, and elevated temperatures, the reactivity of thiophene on Ni2P/SiO2 catalysts increased, producing adsorbed butenes and saturated hydrocarbon fragments. Coadsorption experiments indicate that CO and thiophene compete for the same sites on the Ni2P/SiO2 catalysts, as preadsorbed CO blocked the adsorption of thiophene on Ni2P sites. The IR spectroscopic data for the adsorption and reactivity of thiophene on reduced and sulfided Ni/SiO2 and Ni2P/SiO2 catalysts correlate well with the trend of HDS activity of the catalysts. Specifically, the Ni2P/SiO2 catalysts are more reactive toward thiophene than reduced and sulfided Ni/SiO2 catalysts in UHV and in an atmospheric pressure flow reactor. The increased reactivity of thiophene on Ni2P/SiO2 catalysts in UHV may explain the high turnover frequency previously reported for Ni2P/SiO2 catalysts relative to sulfided Ni/SiO2, Mo/SiO2, and Ni-Mo/SiO2 catalysts.