The solubilities of He, H 2 , N 2 , O 2 , CO 2 , CH 4 , C 2 H 6 , C 3 H 8 , and n-C 4 H 10 were determined at 35ยฐC and pressures up to 27 atmospheres in a systematic series of phase separated polyether-polyamide segmented block copolymers containing either 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. Sorption isotherms are linear for the least soluble gases (He, H 2 , N 2 , O 2 , and CH 4 ), convex to the pressure axis for more soluble penetrants (CO 2 , C 3 H 8 , and n-C 4 H 10 ) and slightly concave to the pressure axis for ethane. These polymers exhibit high CO 2 /N 2 and CO 2 /H 2 solubility selectivity. This property appears to derive mainly from high carbon dioxide solubility, which is ascribed to the strong affinity of the polar ether linkages for CO 2 . As the amount of the polyether phase in the copolymers increases, gas solubility increases. The solubility of all gases is higher in polymers with less polar constituents, PTMEO and PA12, than in polymers with more polar PEO and PA6 units. CO 2 /N 2 and CO 2 /H 2 solubility selectivity, however, are higher in polymers with higher concentrations of polar repeat units. The sorption data are complemented with physical characterization (differential scanning calorimetry, elemental analysis, and wide angle X-ray diffraction) of the various block copolymers.