We have used in situ ATR-FTIR measurements to provide estimates of the strength of specific interactions between carbon dioxide (CO2) and carbonyl groups in polymers such as poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), poly(lactide) (PLA) and poly(lactide-co-glycolide) (PLGA). Polymer films were exposed to high pressure CO2 and the carbonyl stretching vibration at 1700 cm(-1) and the CO2 bending mode at 660 cm(-1) were studied. The observed shift in the carbonyl stretching band to higher wavenumber was attributed to dielectric effects according to the Kirkwood-Bauer-Magat (KBM) equation. On the other hand, the splitting of the CO2 bending mode provided direct evidence of specific interactions between the polymer and CO2. These interactions were quantified via an equilibrium constant for the association reaction between CO2 and the carbonyl group. (C) 2010 Elsevier B.V. All rights reserved.