The permeation properties of H-2, N-2, and CO2 were determined at 35 degrees C and pressures up to 15 atm in phase-separated polyether-b-polyamide segmented block copolymers. These polymers contain poly(ethylene oxide) [PEO] or poly(tetramethylene oxide) [PTMEO] as the rubbery polyether phase and nylon-6 [PA6] or nylon-12 [PA12] as the hard polyamide phase. Extremely high values of polar (or quadrupolar)/nonpolar gas selectivities, coupled with high CO2 permeability coefficients, were observed. CO2/H-2 selectivities as high as 9.8 and CO2/N-2 selectivities as high as 56 were obtained in polymers with CO2 permeability coefficients of approximately 220 x 10(-10) cm(3)(STP) cm/(cm(2) s cmHg). As the amount of polyether increases, permeability increases. Gas permeability is higher in polymers with less polar constituents, PTMEO and PA12, than in those containing the more polar PEO and PA6 units. CO2/N-2 and CO2/H-2 selectivities are higher in polymers with higher concentrations of polar groups. These high selectivity values derive from large solubility selectivities in favor of CO2. Because CO2 is larger than H-2 and has, therefore, a lower diffusion coefficient than H-2, the weak size-sieving ability of the rubbery polyether phase, which is the locus of most of the gas permeation, also contributes to high CO2/H-2 selectivity.