Bromobisphenol A trimethylsilylated polysulfone (BTMSPSf) was synthesized, and the effect of bromo and trimethylsilyl groups on the pure CO₂ and CH₄ transport properties of polysulfone was examined. The ideal separation factor for BTMSPSf is reduced by about 10% than that for unmodified polysulfone (PSf), but BTMSPSf is about two times more permeable than PSf. The effect of the substituents on chain packing was related to the gas permeation properties. Fractional free volume (FFV) calculation, d-spacing and cohesive energy density were used to judge chain packing. In comparison with PSf, the higher values of permeability coefficients for BTMSPSf are due to higher FFV and d-spacing. The small decrease in ideal separation for BTMSPSf is explained as follows; the potential increase in FFV due to packing-disrupting bulky trimethylsilyl groups is overridden by the increase in cohesive energy density attributed to the addition of bromo substituents.