Abstract: Asymmetric gas separation membranes having graded density skins and macrovoid-free morphology comprised of glassy, hydrophobic polymers are disclosed which are effective for separation gases with significante increases in permeation while maintaining equal or greater separation electivity. The membranes have increased free volume and the graded density skin exhibits a density gradient which becomes more dense with increasing proximity to the surface, the membrane effectively decouples the interdependency between permeability and separation selectivity.