The nonideal behavior of polymeric membranes during separation of gas mixtures can be quantified via the solution-diffusion theory from experimental mixed-gas solubility and permeability coefficients. In this study, CO₂-CH₄ mixtures were sorbed at 35 °C in 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA)-m-phenylenediamine (mPDA)-a polyimide of remarkable performance. The existence of a linear trend for all data of mixed-gas CO₂ versus CH₄ solubility coefficients-regardless of mixture concentration-was observed for 6FDA-mPDA and other polymeric films; the slope of this trend was identified as the ratio of gas solubilities at infinite dilution. The CO₂/CH₄ mixed-gas solubility selectivity of 6FDA-mPDA and previously reported polymers was higher than the equimolar pure-gas value and increased with pressure from the infinite dilution value. The analysis of CO₂-CH₄ mixed-gas concentration-averaged effective diffusion coefficients of equimolar feeds showed that CO₂ diffusivity was not affected by CH₄. Our data indicate that the decrease of CO₂/CH₄ mixed-gas diffusion, and permeability selectivity from the pure-gas values, resulted from an increase in the methane diffusion coefficient in mixtures. This effect was the result of an alteration of the size sieving properties of 6FDA-mPDA as a consequence of CO₂ presence in the 6FDA-mPDA film matrix.