Steady-state electron paramagnetic resonance (EPR) spectroscopy using nitroxide spin probes has been used to investigate the plasticization of poly(vinyl acetate) and poly(ethyl methaerylate) by carbon dioxide. By varying the CO2 pressure at constant ambient temperature, the glass transition for each polymer could be depressed to 25 degrees C. This effect has been quantified by a parameter P-50G, obtained by plotting the EPR spectral width as a function of CO2 pressure. Certain spin probes showed free volume distribution effects, manifested in the EPR spectra as "double peaks." Possible reasons for this phenomenon are presented and discussed, and the efficacy of CO2 as a plasticizer is clearly demonstrated by direct comparison with din-butyl phthalate. (c) 2005 Wiley Periodicals, Inc.