The interactions between a variety of carbonyl compounds (ketones, esters, and amides) and dense phase CO2 have been investigated by in situ high pressure FTIR measurements. The gas and liquid phase spectra of these compounds have also been measured to provide accurate reference data, and these have been compared with those obtained in n-hexane and dense phase CO2 at a variety of pressures. The frequency of the carbonyl IR absorption is a measure of the strength of the carbonyl bond, and in the dense CO2 medium, is in between that observed in the gaseous and liquid phases. The substituent effect on the v(C=O) absorption bands is the same for the gas, liquid, and dense CO2 spectra and it is related to the electron donating nature of the substituents. When solutions of carbonyl compounds in n-hexane are diluted, the v(C=O) absorption bands are blue-shifted. In contrast, these bands in dense CO2 are red-shifted with increasing pressure (dilution by CO2). The red shift with increasing CO2 pressure is larger in amides > esters > ketones. (c) 2008 Elsevier B.V. All rights reserved.