The gas transport properties of poly(methyl methacrylate) (PMMA) membranes were examined as a function of physical aging time. A clear decrease of the gas permeability coefficient was observed. Interpretation of the permeation properties using results from differential scanning calorimeter and dynamic mechanical analyzer indicated that the smaller segmental motion of the polymer chains would be more restrict with aging time, discriminating against molecules with decreasing free volume. Free volume variations of PMMA membranes as a function of aging time was investigated by means of positron annihilation lifetime spectroscopy. It is indicated that there is obviously change in the fractional free volume in an aged membrane. The trend of fractional free volume changes consistent with the 13 variation. These results con-elated well with gas transport properties of membranes. Physical aging reduced the measured solubility coefficients and calculated diffusivity coefficients, which was shown to be consistent with decreasing of the free volume. Both diffusivity and solubility selectivity decrease with increasing aging time, however, the effect of physical aging on selectivity coefficients of polymeric membrane was controlled by the change of diffusivity selectivity. (c) 2007 Elsevier B.V. All rights reserved.