Mixed-gas sorption of CO2-CH4 mixtures in rubbery polydimethylsiloxane (PDMS) at 35 °C demonstrated that the presence of CH4 changed the behavior of CO2 sorption and vice versa. This mutual interaction indicated that gases in mixtures do not sorb independently in rubbery membranes. Moreover, we observed that at increasing pressures the interaction between PDMS and CO2-CH4 mixtures enhanced the solubility selectivity of PDMS. Mixed-gas solubility coefficients of CH4 in PDMS were lower than 0.5 cm3(STP) cm−3 atm−1. To accurately measure these values, a new sorption system was designed, constructed, and optimized for low solubility coefficients; an operator-friendly approach to mixed-gas sorption experiments is also discussed in this work. CO2-CH4 mixed-gas diffusivity trends were evaluated from Maxwell-Stefan model fitting of mixed-gas permeation and sorption data. The analysis of both mixed-gas diffusion and sorption data demonstrated that CO2/CH4 mixed-gas permselectivity of PDMS was mainly influenced by CO2 sorption. In mixtures, CH4 diffusion coefficients increased with higher volumetric CO2 concentration, whereas CO2 diffusion coefficients were essentially concentration independent in both pure- and mixed-gas environments.