Polysulfone copolymers based on mixtures of bisphenol A, BPA, and 1,1 bi-2 naphthol, BN, diols have been synthesized and their gas permeability coefficients and selectivity separation factors for O-2/N-2 and CO2/CH4, at 5 atm and 35 degreesC, have been measured in a standard permeation cell. The polysulfone copolymers can form flexible thin films suitable for gas separation membranes. The gas selectivity for O-2/N-2 measured for the polysulfone copolymers synthesized with 50 and 70 mol % of BN, with the rest being BPA, in the initial mixture of diols are 6.4 and 6.8, respectively. The corresponding gas permeability coefficients for O-2 are 1.24 and 1.09 Barrers. Compared to the corresponding selectivity and permeability balance reported for polysulfones based on pure BPA, BPA-PSF, the copolymers show a balance that moves in the direction of higher selectivity with small losses in the permeability of the fastest gas. From the glass transition temperature determinations, it is observed that the incorporation of BN in the repeating unit of BPA-PSF inhibits large-scale segmental motions that are reflected in reductions in the diffusivity coefficients for all gases. (C) 2003 Wiley Periodicals, Inc.