AbstractDilation of polysulfone (PSUL) and crystalline poly(ethylene terephthalate) (PET) films accompanying sorption of carbon dioxide is measured by a cathetometer under high pressure up to 50 atm over the temperature range of 35–65°C. Sorptive dilation isotherms of PSUL are concave and convex to the pressure and concentration axes, respectively, and both isotherms exhibit hysteresis. Each dilation isotherm plotted versus pressure and concentration for the CO2‐PET system shows an inflection point, i.e., a glass transition point, at which the isotherm changes from a nonlinear curve to a straight line. Dilation isotherms of PET below the glass transition point are similar to those of the CO2‐PSUL system, whereas the isotherms above the glass transition point are linear and exhibit no hysteresis. Partial molar volumes of CO2in these polymers are determined from data of sorptive dilation. On the basis of the extended dual‐mode sorption model and the current data, primitive equations for gas‐sorptive dilation of glassy polymers ar