A liquid-phase reaction cell, coupled to an ultrahigh-vacuum surface analysis chamber, was built to study liquid-phase hydrogenation reactions on small area model catalyst surfaces. The hydrogenation of cyclohexene to cyclohexane was studied at about 1.5 atm total pressure of H-2 and as a function of temperature in the 313-333 K range in liquid cyclohexene. The hydrogenation rate of cyclohexene on a clean Pt surface increased with the increasing circulation velocity of the liquid, indicating that the reaction rate was controlled by hydrogen diffusion to the surface. When the surface reaction rate was reduced by the deposition of hydrocarbon fragments on the platinum surface, the rate of cyclohexene hydrogenation became independent of hydrogen diffusion and became controlled by the kinetics of the surface reaction. The estimated activation energy of the reaction is 6 kcal/mol for cyclohexene hydrogenation on a platinum foil that was partly covered with carbonaceous deposits.