A theory based on taking into account structural rearrangements of the polymer matrix is used to analyze hysteresis effects that have been observed during a penetrant absorption-desorption cycle in a glassy polymer. A previously developed theoretical description for the absorption process is used as a basis for developing equations describing volumetric and isotherm data for the desorption process. A different sample history is used for desorption experiments, and this difference in sample history leads to different volumetric and sorption behavior. The theoretical predictions are compared with volumetric and isotherm data for the glassy polycarbonate-carbon dioxide system.