Polyarylates based on isophthalic (IA) and terephthalic (TA) acids and an equimolar mixture of the diols Bisphenol A (BPA) and 1,1 bi-2-naphthol (BN) were synthesized to produce BPA-BN/IA and BPA-BN/TA polymers and to measure their gas permeability coefficients, P(i), at several pressures and 35 degrees C, to the gases O-2, N-2, CH4, and CO2. For the BPA-BN/IA membranes, at a 2 atm up-stream pressure, the P(O-2) and P(CO2) are 0.93 and 4.0 Barrers with O-2/N-2 and CO2/CH4 ideal separation factors of 6.7 and 27. For the BPA-BN/TA, at a 2 atra up-stream pressure, the P(O-2) and P(CO2) are 2.0 and 9.9 Barrers with O-2/N-2 and CO2/CH4 ideal separation factors of 5.6 and 21. Comparing the selectivity-permeability balance of properties shown by the BPA/TA membranes with that shown by the copolymer BPA-BN/TA, the balance moves in the direction of higher selectivity and lower permeability because of the incorporation of BN, which is a more rigid monomer than BPA. However, when the balance of properties for the pair O-2/N-2 shown by BPA-BN/TA is compared with the one shown by other membranes such as those based on mixtures of diols and diacids, that is the bisphenol A-naphthalene/I-T polymers reported in the literature, the balance moves up and to the right of the typical selectivity-permeability trade-off observed in the BPA-polyarylate family. Thus, simultaneous incorporations of flexible and rigid monomers in both the diols and the diacids lead to more productive and more selective membranes. (c) 2005 Wiley Periodicals, Inc.