The interactions between CO2 and carbonyl compounds at different CO2 pressures have been studied both experimentally and theoretically. In situ high-pressure FTIR on carbonyl compounds, i.e., acetaldehyde, acetone, and crotonaldehyde, in supercritical CO2 have been measured at various CO2 pressures varying from 6 to 22 MPa. In order to get insights into the mechanism, theoretical study has been conducted concerning the effect of CO2 on frequency shift of C = O in acetaldehyde, acetone, benzaldehyde, crotonaldehyde and cinnamaldehyde at different CO2 pressures. It has been shown that the experimental frequency shifts can be well simulated by the theoretical model calculations using particular structures, in which a carbonyl compound interacts with a few CO2 molecules, depending on the carbonyl compounds examined, except for acetaldehyde. The interaction energies between CO2 and those carbonyl compounds are also given. In addition, the effect of CO2 on hydrogenation of crotonaldehyde and benzaldehyde has been discussed by means of the local softness (s(+)) calculated at CO2 pressures of 0-22 MPa, which can explain the reactivity difference in the crotonaldehyde and benzaldehyde hydrogenations in supercritical CO2. (C) 2010 Elsevier B.V. All rights reserved.