Copoly(ether imide)s formed by the reaction of an aromatic dianhydride (BPDA), an aromatic diamine (ODA or BNZ), and alpha,omega-bis(2-aminopropyl polypropylene oxide)s with molecular weights of 2000 and 4000 g/mol were obtained, characterized, and studied as gas separation membranes. All of the copoly(ether imide)s studied showed a low imidization temperature, well below 200 degrees C. They also underwent a phase segregation process, as confirmed by SAXS, when they were thermally treated. The segregation was found to increase permeability without any significant decrease of selectivity for almost all of the gas pairs studied (O-2/N-2, CO2/CH4, and CO2/N-2). For the CH4/N-2 pair, a strong increase in selectivity was obtained upon segregation, along with a rising permeability. This excellent gas productivity for the mixture CH4/N-2 is probably due to a combined effect of a specific interaction and an increase in solubility of CH4 in the soft segregated domains of the copolymer. Methane permeabilities over 20 barrer and CH4/N-2 selectivities close to 4.2 for an applied pressure of 1 bar were attained. The changes in permselectivity with permeation temperature were observed to follow an Arrhenius correlation, which adds a certain tunable permeability versus selectivity to these materials. Thus, it is possible to attain high selectivities (for relatively low temperatures) or high permeabilities (for comparatively high temperatures). The proportion of PPO in the final polymer has a significant influence on the segregation extent and domains, so that both the selectivity and permeability improved when the proportion of polyether in the final polymer was high. The mechanical properties were good for all of the proportions and/or densities tested. All of the copolymers studied showed a high resistance to plasticization and to the action of solvents and aggressive media. The high permselectivities and resistance to plasticization and solvents suggest that the membranes obtained could be advantageously used in natural gas extraction and purification.