Liquid-phase hydrogenation of cyclohexene under mild conditions on Pd/SiO2 in different organic solvents (benzene, heptanol, and NMP), under pressurized carbon dioxide, and under solvent less condition were investigated and compared. In the cases of using organic solvents, the hydrogenation rates depended on polarity of the solvents in which the reaction rates in high polar solvents such as heptanol and NMP were lower than that in a non-polar solvent. Hydrogenation rates were much higher when the reactions were performed under high-pressure CO2 or under solvent less condition. The use of high-pressure CO2 can probably enhance H-2 solubility in the substrate resulting in a higher hydrogenation activity. However, metal sintering and leaching in the presence of high-pressure CO2 were comparable to those in organic solvents. (c) 2006 Elsevier B.V. All rights reserved.