Gas permeability coefficients were obtained for CO2 and He gases at room temperature in a semicrystalline ethylene/ethylene-propylene/ethylene (E/EP/E) triblock copolymer, and a blend of polyisoprene (PI) with an E/EP diblock copolymer. It was found that the gas transport properties of these polymer systems were influenced by changes in processing conditions and solvent treatments to create nanoporous membranes. The permeability results coupled with small angle X-ray scattering (SAXS) data, provide a direct connection between changes in microstructure to the observed changes in gas transport properties. Channel die processing was used to alter the relationship of semicrystal-line and microphase separated lamellar populations and produce orientation textures applicable for the construction of membranes exhibiting anisotropic gas transport properties. Depending on the processing conditions, these microstructures can be oriented normal to the plane of shear and perpendicular or transverse to the direction of shear. It was demonstrated that the transverse crystalline lamellar morphology;exhibits higher gas permeability than the perpendicular microphase separated orientation morphology due to its lower crystallinity. E/EP/E triblock samples were also processed in the channel die, and then exposed to xylene or heptane to create nanoporous membranes. Depending on the choice of solvent, various degrees of porosity could be induced in the polymeric membrane. The microstructure orientation texture produced from channel die processing was not disrupted by the introduction of the pores. An E/EP randomly oriented diblock copolymer was blended with PI homopolymer and then exposed to a selective solvent to extract the homopolymer and create a porous microstructure. It was shown that the permeability of the block copolymer membrane increased due to the introduction of pores, while retaining some selectivity due to the confinement of the pores within the block copolymer self-assembled microphase separated morphology. (C) 2000 Elsevier Science Ltd. All rights reserved.