The gas permeation behavior of hexafluoro-dianhydride (6FDA)-durene/3,3'-diaminodiphenyl sulfone (DIDS) polyimides was investigated by systematically varying the diamine ratios. Generally, the gas permeability, diffusion and solubility coefficients were found to decrease with increasing 3,3'-DDS diamine content. The permeability coefficients of H-2, O-2, N-2 and CO2 decreased with the increasing order of kinetic diameters of the penetrant gases. However, the decrease in diffusion coefficients of O-2, N-2 and CO2 was in accordance with the effective diameters of the gas molecules. The permselectivity of gas pairs such as CO2/N-2, O-2/N-2 and H-2/N-2 was enhanced with the incorporation of 3,3'-DDS moiety. The gas transport coefficients of the copolyimides predicted from the addition rule were compared with the experimental results. It was seen that gas permeation coefficients for the copolyimides slightly deviated from the predicted values. In contrast, the logarithm of gas permeation coefficients and the reciprocal of fractional free volume (1/FFV) exhibited a better correlation.