The effect of dissolved carbon dioxide on the glass transition temperature of a polymer, PMMA, has been investigated using molecular probe chromatography. The probe solute was iso-octane, and the specific retention volumes of this solute in pure PMMA and mixtures of PMMA with CO2 were measured over a temperature range of 0 to 180 degrees C and CO2 pressures from 1 to 75 atm. The amount of CO2 dissolved in the polymer was calculated from a model fit to previously published solubility data determined chromatographically. Classical van＇t Hoff-type plots were used to determine the glass transition temperature of CO2-impregnated PMMA from low pressure up to 46 atm of CO2. Solvent-induced plasticization was observed with the glass transition temperature decreasing by about 40 degrees C. At some pressures, glass transitions at low temperatures could not be determined from the van＇t Hoff plots because of the proximity of the polymer glass transition temperature to the gas-liquid transition temperature for CO2. For these pressures, a new method was developed to determine the glass transition composition. The glass transition pressure was then calculated from the measured composition and temperature using an isotherm model. In every case, the glass transition temperature decreased linearly with increasing concentration of CO2 in the polymer. However, at higher compositions, the glass transition pressure decreased with increasing composition and decreasing temperature. The observed retention volume of iso-octane with PMMA in a glassy state was correlated with an adsorption model developed from a theory for liquid-solid chromatography derived by Martire. This model accurately described the observed decrease in retention of iso-octane by adsorption on the surface of glassy PMMA with increasing concentration of CO2 dissolved in the polymer.