Free-volume properties, size and distribution, in amorphous polystyrene exposed to CO2 gases have been measured as a function of pressure to 800 psi (5.5 MPa), of time, and of temperature using positron annihilation lifetime spectroscopy. The free volume increases significantly and its distribution broadens as a function of pressure. The free volume relaxes as a function of time with a characteristic time of 15 h, and 5.7 h for 400, and 800 psi, respectively, after depressurizing under vacuum. A portion of free volume created by CO2 exposure remains permanently in the polymer after CO2 exposure. The glass transition temperature decreases significantly as a function of CO2 pressure from the free-volume data and is compared with the differential scanning calorimeter results. The observed free-volume variations as a function of pressure, time, and temperature are discussed in terms of hole expansion, creation, free-volume relaxation, plasticization, and hole filling in amorphous polymers. (c) 2008 Wiley Periodicals, Inc.