The transport properties of CO₂ and CH₄ for TMSPSf (bisphenol A trimethylsilylated polysulfone) were measured, and compared with the values for PSf (bisphenol A polysulfone) and MPSf (bisphenol A methylated polysulfone) to explain the effect of molecular structure of polysulfones on gas transport properties. The permeability coefficients of three polysulfones rank in the order : TMSPSf>PSf>MPSf. TMSPSf is several times more permeable than PSf. The effect of the substituents on chain packing was related to the gas transport properties. The ranking of permeability coefficient correlates well with fractional free volume. The variation of d-spacing is also reasonably consistent with the permeability coefficient. The effects of pressure on the sorption and permeation properties of polysulfones were examined. The permeation properties for a mixture of CO₂ and CH₄ were also measured and these results were compared with the values of pure gases. The sorbed concentrations and permeability coefficients are well fitted to dual mode model. The permeability coefficients of each gas of binary mixture are reduced than those for pure gases, and the extent of reduction in permeability coefficient is the smallest for TMSPSf, which has the highest value of Langmuir capacity constant.