The hydrogenation of benzaldehyde and cinnamaldehyde has been studied with a 5% Pt/C catalyst in compressed CO2. The effect Of CO2 pressure on the total conversion was found to be different between the two aldehydes. The total conversion of benzaldehyde merely decreases with increasing CO2 pressure, while that of cinnamaldehyde shows a maximum at a certain pressure. High-pressure FTIR measurements indicate the existence of interactions Of CO2 with the aldehydes. The absorption of v(C=O) red-shifts at increasing CO2 pressure, and this red-shift is more significant for cinnamaldehyde than for benzaldehyde, indicating that the C=O bond of the former becomes more reactive than the latter. The difference in the mode of interactions Of CO2 with these aldehydes has also been indicated by changes of v(C=O) Of CO2. Thus, the conversion of benzaldehyde will decrease with increasing CO2 pressure because of a simple dilution by introducing a larger quantity Of CO2. For cinnamaldehyde, the conversion will increase at low pressures because of increasing interactions with CO2 Molecules (increasing the reactivity of the C=O bond) but decrease at high pressures because of the simple dilution effect, similar to the case of benzaldehyde. The dense CO2 molecules are not likely to change the catalytic activity of supported Pt particles, which was previously suggested from optical absorption of supported fine metal (An) particles in a compressed CO2 medium.