Hydrogenation of phenol to cyclohexanone and cyclohexanol in/under compressed CO2 was examined using commercial Rh/C and Rh/Al2O3 catalysts to investigate the effects of CO2 pressurization on the total conversion and the product selectivity. Although the total rate of phenol hydrogenation with Rh/C was lowered by the presence of CO2, the selectivity to cyclohexanone was improved at high conversion levels >70%. On the other hand, the activity of Rh/Al2O3 was completely lost in an early stage of reaction. The features of these multiphase catalytic hydrogenation reactions using compressed CO2 were studied in detail by phase behavior and solubility measurements, in situ high-pressure FTIR for molecular interactions of CO2 with reacting species and formation/adsorption of CO on the catalysts, and simulation of reaction kinetics using a simple model. The CO2 pressurization was shown to suppress the hydrogenation of cyclohexanone to cyclohexanol, improving the selectivity to cyclohexanone. The formation and adsorption of CO were observed for the two catalysts at high CO2 pressures in the presence of H-2, which was one of important factors retarding the rate of hydrogenation in the presence of CO2. It was further indicated that the adsorption of CO on Rh/Al2O3 was strong and caused the complete loss of its activity. (C) 2010 Elsevier B.V. All rights reserved.