An increasing number of studies have been reported on the use of supercritical fluids to process polymers. The presence of impurities such as residual solvents, unreacted monomers, catalysts, and side reaction products can adversely affect the end-use properties of polymeric materials. Therefore, these impurities must be reduced to a level below the maximum permissible limit. Conventional devolatilization techniques for the purification of polymers have limited effectiveness. Devolatilization with supercritical fluids, however, can enhance impurity removal by increasing the thermodynamic driving force and molecular diffusivity. A model for supercritical devolatilization developed incorporates swelling of the polymer film and convection induced by the diffusion that arises because of the nonideal volumetric behavior. The perturbed soft-chain theory is used to predict the nonideal thermodynamic behavior. Free volume theory is utilized to predict the influence of plasticization and relaxation on impurity mass transfer. The removal of benzene from polystyrene film with supercritical carbon dioxide was simulated. This analysis, which is consistent with experimental results in the literature, indicates that, contrary to conventional wisdom, impurity extraction can be dominated by mass transfer during the quick depressurization step at the end of a supercritical-fluid extraction process.