Free volume of polystyrene films treated with supercritical carbon dioxide (SCCO2) were examined by positron annihilation lifetime spectroscopy. Variation of the free volume sizes after the SCCO2 treatment due to the release of trapped CO2 and structural relaxation of the polymer was observed. After 500 h from the depressurization, the free volume of the treated films free from CO2 was similar to 0.306 nm in radius and much larger than that of the untreated films of similar to 0.295 nm in radius due to the freezing of the swollen structure caused by the large solubility of CO2.