The sorption and permeation properties of the CO₂ and CH₄ were measured for polysulfone and dimethylated polysulfone to investigate the structure-property relationships. The effect of operating pressure on the transport properties of the polysulfones was examined. The permeation properties for a mixture of CO₂ and CH₄ (CO₂/CH₄=57.5/42.5 vol%) were also measured and these results were compared with those obtained from the experiments of pure gases. The sorptions of CO₂ and CH₄ are well described by "dual-sorption model". The permeability coefficients of CO₂ and CH₄ decreases with increasing upstream pressure, as is often the case with other glassy polymers. The permeability coefficients of each gas of binary mixture are reduced than those for pure gases. This result is due to the competition of each gas for the Langmuir sites. The free volume of the dimethylated polysulfone is lower than that of polysulfone, and dimethylated polysulfone shows relatively lower permeability coefficients and higher selectivity than polysulfone.