Activity and product selectivity have been measured for thiophene hydrodesulfurization (HDS) over model silica-supported molybdenum catalysts at a pressure of 1 atm and at temperatures ranging from 398 to 673 K. The model catalysts feature isolated molybdenum atoms in the +2, +4, and +6 oxidation states and molybdenum dimers with each molybdenum atom in the +4 oxidation state. Silica-supported MoS2, prepared by deposition and decomposition of (NH4)(2)MoS4, was used for reference. There is a correlation between thiophene HDS activity and molybdenum oxidation state, with Mo(II) most active. Thiophene I-IDS activity does not show a significant structure dependence for isolated Mo(IV) versus dimeric Mo(IV) catalysts, Activation energies of 51.5 and 49.9 kJ/mol were determined for thiophene I-IDS over Mo(II) and MoS2/SiO2 catalysts, respectively. Butane and butenes are the major products of thiophene HDS with little butadiene detected. Activity and selectivity trends suggest the HDS reaction is initiated by eta(1) binding of thiophene on the supported metal catalysts.