In this study, the microstructural changes in polycarbonate (PC) membranes induced by the CO2 sorption were investigated using the CO2 sorption isotherm data and the positron annihilation lifetime spectroscopy (PALS). The effects of the PC exposure to CO2 at different temperatures and gas pressures on the free-volume distribution were discussed. The distribution at ambient conditions or 5 atm He pressure displayed a monomodal curve, whereas that at different CO2 pressures showed a bimodal curve, indicating the presence of small and big free-volume sizes due to the serious plasticization caused by CO2. (C) 2010 Elsevier B.V. All rights reserved.