Pure and mixed gas n-C 4 H 10 and CH 4 sorption and dilation properties in poly(dimethylsiloxane) (PDMS) are reported at temperatures ranging from −20 to 50°C. CH 4 partial pressures range from 3.1 to 16atm, and n-C 4 H 10 partial pressures range from 0.07 to 1.3atm. Based on the mixed gas sorption results, the presence of n-C 4 H 10 in the polymer enhances CH 4 solubility. For instance, at 25°C, a 10% increase in CH 4 solubility, from 0.50 (pure gas) to 0.55cm 3 (STP)/(cm 3 polymer atm), is observed when the n-C 4 H 10 activity goes from 0 to 0.29. At −20°C, as n-C 4 H 10 activity increases from 0 to 0.53, CH 4 solubility increases by almost 35%, going from 0.77 to 1.03cm 3 (STP)/(cm 3 polymer atm). In contrast, n-C 4 H 10 sorption properties in these mixtures are not measurably affected by the presence of CH 4 . The n-C 4 H 10 /CH 4 solubility selectivity in PDMS increases as temperature decreases and as n-C 4 H 10 activity in the mixture increases. For example, at 25°C, the solubility selectivity increases from 52 to 60 as n-C 4 H 10 activity increases from 0.11 to 0.34. At 0°C, n-C 4 H 10 /CH 4 solubility selectivity increases from 84 to 105 as n-C 4 H 10 activity increases from 0.09 to 0.33. Pure and mixed gas dilation results are used to calculate partial molar volumes of CH 4 and n-C 4 H 10 in the polymer. Dilation isotherms in mixtures agree with estimates based on pure gas sorption and dilation measurements.