A complete series of aliphatic-aromatic copoly(ether-imide)s has been synthesized in this work. All these copoly(ether-imide)s had the same structure, BPDA-PEO-ODA, but different lengths of PEO in the final polymer. These copolymers have been thermally treated and characterized by several techniques. A direct relationship between the temperature of treatment, the improvement of phase segregation, and permeability has been demonstrated. The Maxwell model has been applied to predict permeability (for CO2, CH4 and N-2) and it has been found that when the segregated PEO can be considered to be amorphous - it is at high temperatures when crystallinity disappears - the model fits reasonably well. This confirms that the aliphatic and aromatic portions of the copolymer behave approximately as a bi-phase of disperse domains within a continuous matrix. Results show that permeability is higher when the PEO chains are longer - when there is no crystallinity, or any kind of internal bonds, distorting the results - while selectivity does not depend on the PEO length in the copolymer. Remarkable are the results for the CO2/N-2 gas pair, with selectivity-versus-permeability very near to the Robeson's upper bound at 30 degrees C and even in closer proximity to the corresponding trade off line for higher permeation temperatures. (C) 2013 Elsevier Ltd. All rights reserved.