A set of copoly(ether-imide)s, formed by the reaction between an aromatic dianhydride (BPDA), a poly-oxyalkyleneamine (poly( ethylene oxide) diamino terminated) (PEO-2000) and various aromatic diamines (PPD, BNZ, and ODA) under different thermal treatments, has been obtained and characterized. The existence of phase segregation in these copolymers has been confirmed by SAXS. It has also been proved that aromatic diamines having a hinge group, such as ODA, produce bigger segregated domains while short and rigid aromatic diamines give higher percentages of phase segregation. A direct relationship between gas permeability and percentage, and size of the domains, of the segregated phase has been obtained. Also, higher or similar selectivities have been observed as a function of the thermal treatment. In such way, the use of high temperatures during the thermal treatment, responsible of the improvement of the phase segregation, enhances the gas properties, producing materials with permeability/selectivity values close to the Robeson limit. In the case of the CH4/N-2 pair an interesting reverse selectivity has been observed probably due both to a specific interaction of the gas with the polymer and to an increase in the solubility of CH4 in the soft segregated portions of the copolymer. The use of PPD as comonomer provides more phase segregation while the use of ODA gives bigger segregated domains. In terms of permselectivity the performances, for all the pairs of gases studied, follow the PPD > BNZ > ODA sequence when relatively low treatment temperatures are used while quite similar results for BNZ and ODA were observed when higher treatment temperatures are employed. This seems to indicate that the domains size is a key factor in the gas separation properties with a certain influence of the percentage of segregation. An adequate balance of short length with limited rigidity of the aromatic part should improve the properties of this kind of materials. (C) 2011 Elsevier B.V. All rights reserved.