Copolyimide membranes with different poly(ethylene oxide) (PEO) content (from 28 to 68 wt percent, wt.%) have been thermally treated at different temperatures (from 200 to 300 degrees C) to evaluate the effect of the thermal protocol on the gas transport properties to O-2, N-2, CO2 and CH4. The permeability coefficients (P) for all gases increased after the thermal treatment of the membranes and were related to the PEO content, being this enhancement higher for membranes with lower PEO content. Thermal treatment at 300 degrees C of the membranes with 28 and 43 wt.% of PEO, yielded more productive materials for CO2/N-2 separation since the permeability coefficients for CO2 (P-CO2) increased 9.8 and 3.2 times, respectively, while the selectivity just suffered a small drop (less than 1.3 times in both cases). Overall, the membrane with 43 wt.% of PEO exhibited the best performance, with a P-CO2 of 78 Barter and a CO2/N-2 selectivity of 52. For CO2/CH4 separation, an increase on selectivity of 1.8 times was obtained in the copolyimide with 43 wt.% of PEO, reaching the selectivity a value of 18. This enhancement of productivity has been associated to an improvement of phase segregation. (C) 2008 Elsevier B.V. All rights reserved.