The selective hydrogenation of trans-cinnamaldehyde (CAL) has been investigated with several types of multiphase reaction systems using Ru-phosphine complex catalysts in the presence of high-pressure CO2. The selective formation of an unsaturated alcohol, cinnamyl alcohol (COL), can be achieved in an organic solvent free two-phase system, which includes the liquid (CAL, Ru complex) and gas (H-2, CO2) phases. Both total conversion and COL selectivity are enhanced with increasing CO2 pressure. This enhancement is due to the dissolution of CO2 molecules into the CAL phase. which promotes the dissolution of H-2 and activates the reactivity of the carbonyl group of CAL molecules, and a high concentration of CAL in the reaction medium. The high COL selectivity can also be obtained in three- and two-phase reaction systems, which include a gas (H-2, CO2) phase, a liquid (water-dissolvin- Ru complex) phase, and/or another liquid phase (CAL). The COL selectivity is high irrespective of CO2 pressure because of a water-CAL interface or a water-CO2 interface as a main reaction locus, but the total conversion is not enhanced by pressurization with CO2, and it decreases at elevated CO2 pressure under the two-phase conditions because of a simple dilution effect. In contrast, the highly selective formation of COL is not possible in a homogeneous dense CO2 gas phase or in a two-phase system that includes a gas (H-2) phase and a liquid (DMF, CAL, Ru complex) phase. Pressurization with CO2 is not effective in improving the conversion and COL selectivity for these systems. (c) 2005 Elsevier Inc. All rights reserved.