The sorption and transport properties of pure CO2 and CH4 for a series of polysulfones were measured. The effects of molecular structure of polysulfones on transport properties were studied using chemically modified polysulfones, including TMSPSF (bisphenol-A trimethylsilylated polysulfone), BPSF (bromobisphenol-A polysulfone), and BTMSPSF (bromobisphenol-A trimethylsilylated polysulfone). The effects of operating 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 those obtained from the experiments of pure gases. The sorbed concentrations and permeability coefficients are well fitted to a conventional dual-mode model. The permeability coefficients of each gas of a binary mixture are lower than those of pure gases, which shows the competition effect between each component. The permeability coefficients of polysulfones rank in the following order, TMSPSF > BTMSPSF > bisphenol-A polysulfone (PSF) > BPSF. The effect of the substituents on chain packing was related to the gas-permeation properties. Fractional free volume (FFV) calculations and X-ray diffraction were used to judge chain packing. In comparison with PSF, the higher values of permeability coefficients for TMSPSF and BTMSPSF are due to higher FFV and d spacing. The lower permeability coefficients for BPSF is attributed to the strong induced dipole interchain interaction. Addition of bromo substituents to TMSPSF is also found to decrease the permeability coefficients for BTMSPSF, suggesting that the potential increase in FFV due to packing-disrupting bulky trimethylsilyl groups is overridden by the increase in cohesive energy density. (C) 2000 John Wiley & Sons, Inc.