Poly(ether imide) segmented copolymers were prepared from polyether-diamine and comonomer diamine with acid anhydride. They have microphase-separated structures consisting of microdomains of rubbery polyether segments and of glassy polyimide segments. The gas permeation occurs through the polyether segment microdomains, and the polyimide segment microdomains contribute to mechanical properties and film-forming ability. The poly(ethylene oxide) (PEO)-based copolymers effect very high CO2/N-2 separation, for example, permeability coefficient to CO2 P-CO2 = 1.4 x 10(-8) cm(3) (STP) cm(2)/(cm s cmHg) and permeability ratio of CO2 over N2PCO2/P-N2, = 70 at 298 K. This high permselectivity is attributed to high solubility selectivity due to the affinity of CO2 to PEO segments. On crystallization of PEO segment microdomains, P-CO2 decreases but P-CO2/P-N2, does not change. The effects of PEO content, PEO block length, kind of acid anhydride and diamine, temperature, and feed pressure on P-CO2 and P-CO2/P-N2 are investigated.