Sum frequency generation surface vibrational spectroscopy and kinetic measurements using gas chromatography have been used to identify a reactive surface intermediate in situ during hydrogenation of benzene on a Pt( 111) single crystal surface at Torr pressures. Upon adsorption at 310 K, both chemisorbed and physisorbed benzene coexist on the surface, a result which has not previously been observed. Kinetic measurements show a linear compensation effect for the production of both cyclohexane and cyclohexene. From these data the isokinetic temperature was identified and correlated to the chemisorbed benzene species, which were probed by means of vibrational spectroscopy. Additionally, chemisorbed benzene was determined to be a reactive intermediate, which is critical for hydrogenation.