The effect of polycarbonate (PC) membranes treated with super critical carbon dioxide on gas sorption and transport properties was studied. It was found that the gas permeability of PC membranes hardly changed after the CO2 treatment. However, the gas sorption and diffusion properties showed a significant change. The transport properties were determined by measuring the sorption and permeation properties. The oxygen and nitrogen sorption measurements were carried out at 35 degrees C over the pressure range of 1-25 atm. It was found that exposed CO2 treatment hardly changed the gas permeability but slightly increased the selectivity of oxygen to nitrogen. This study also showed that the increase in gas solubility was contributed by increasing Langmuir-type sorption of CO2 treated membranes rather than Henry-type sorption. The change in polymer packing of membrane was evidenced by glass transition temperature measurements. This study showed that the free volume of membranes increased with the increase of exposed CO2 density.