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SynopsisSorption of carbon dioxide, ethylene, and nitrous oxide in poly(methy1 methacrylate) (PMMA) at 35°C has been characterized for each gas as a pure component and for mixtures of carbon dioxidelethylene and carbon dioxide/nitrous oxide. Pressures up to 20 atm were examined. Pure-component sorption isotherms are concave to the pressure axis for each of the gases. This behavior is accurately described by the dual-mode sorption model. Using only the p u r e component dual-mode parameters and the generalization of the model for gas mixtures, one can predict the total concentration of gas sorbed in the polymer to within an average deviation of *2.01% for the C02/C2H4/PMMA system and f0.98% for the C02/NzO/PMMA system. In both systems, for each component of the mixture, sorption levels were lower than corresponding purecomponent sorption levels at pressures equal to the partial pressure of the respective components in the mixture. Depression of the sorbed concentration in mixture situations appears to be a general feature of the above systems and can be substantial in some situations. For the CO2/C2H4/PMMA system, use of pure-component sorption data to estimate the total sorbed concentration in the mixture would be in error by as much as 40% if one failed to account for competition phenomena responsible for depression in mixed-gas situations.Mixture pressures as high as 20 atm were studied for both systems and in the C02/N20/ PMMA system sorbed concentrations reach 33.90 [cmYSTP)/cmS polymer] without any significant deviation from model predictions.As for many other gas/glassy polymer systems, the pure-component data in these studies were adequately described by the dual-mode sorption m~del.~-lO Subsequent use of the pure-component parameters allowed prediction of mixture sorption behavior via the dual-mode model for mixed gases. For pure components the model is given by eq. (l), while eq. (2) 'Present address: