The Freeze-Purged-Desorption (FPD) method was developed for the experimental measurement of gas permeability coefficients as a new technique using a desorption curve of gas immobilized in polymeric films. The FPD method was effectively used to evaluate four gas permeation parameters (C-D, C-H, D-D, and D-H,) of glassy polymeric films (polycarbonate and polystyrene) by using CO2. The modes of the CO2 gas desorption response curve (D-curve) obtained were sensitively characterized by the proportion of sorption in the Henry and Langmuir modes in the polymeric films accompanied by their own gas diffusivity. A graphical analysis of the D-curve of CO2 reasonably proposed a linear relation between the desorption rate and the sorption amount of CO2 which was strongly influenced by the kind of sorption gas, film, temperature, and other factors. The desorption rate of sorbed CO2 gas for the PC and PS films gave a characteristic straight line with an inflection point indicating a shift in the gas-diffusion mechanism from the complex type of the Henry and the Langmuir modes to the Langmuir mode. The characteristic D-curves obtained were graphically analyzed, and they clearly discriminated the Henry mode part and the Langmuir mode part. This discrimination process quantitatively and individually evaluated C-D, C-H, D-D, and D-H. By using the four parameters evaluated, a mathematical model to describe the D-curve was proposed, and it consistently explained the experimental D-curves. (C) 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 934-941,2004.