We present the use of supercritical CO2 (scCO(2)) as reaction solvent in a packed bed silicon/glass microreactor in case of the hydrogenation of cyclohexene as a model reaction. In situ phase studies in the continuous flow of the reaction mixture, hydrodynamic characterization and the influence of pressure and temperature on the reaction performance are discussed. The results are compared with the same reaction conducted in three phase gas-liquid-solid state and larger scale reactors using scCO(2) as reaction solvent. The phase study experiments show a single phase flow behavior at 136 bar and 25 degrees C for a 90:5:5 molar mixture of CO2:C6H10:H-2. At 50 degrees C compositions up to 87.8:2.4:9.8 show single phase flow. The reaction performance increases with increasing temperature where pressure variations show no significant change. The comparison with larger scale systems indicates an increase of about one order of magnitude in space time yield in the presented microreactor. (C) 2008 Elsevier B.V. All rights reserved.