Membrane polymers with high permeability and high selectivity are preferred for gas separations. However, there exists a trade-off or upper bound, i.e., polymers with higher permeability often exhibit lower selectivity, and vice versa. The upper bound for separation of various gas pairs has been empirically drawn and theoretically rationalized using pure-gas data. However, for CO2/CH4 separation, the high pressure CO2 and non-methane hydrocarbons can plasticize polymers, increasing mixed-gas CO2 permeability and decreasing mixed-gas CO2/CH4 selectivity. This study aims to apply a modified free volume theory to interpret CO2/CH4 separation performance in polymeric membranes. The model satisfactorily describes the pure-gas upper bounds for various gas pairs including CO2/CH4, the effect of high pressure CO2 on mixed-gas CO2/CH4 separation properties, and the practical mixed-gas upper bound for CO2/CH4 separations. The CO2 is found to have an estimated glass transition temperature of 108 K. The assumptions of this model are discussed, and future work to improve this model is proposed. (C) 2014 Elsevier B.V. All rights reserved.