Recovery of CO2 from gas mixtures is an important industrial objective in view of several aspects such as natural gas reforming, biomedical applications and even global warming. Polyimides consisting of flexible polyethyleneoxide (PEO) segments and rigid polyimide segments have been reported in recent years for this application. They exhibit high CO2 permeability coefficients and relatively high CO2/N-2 selectivity due to CO2 strong affinity to the polar PEO segments. In this work a set of copolyimides with different contents of PEO have been prepared from 3,3',4,4'-biphenyltetracarboxylic dianhydride, oxydianiline (ODA) and a diamine containing the PEO fragment. ODA-PEO ratios (w:w) used to prepare the polymers were 1: 1, 1:2, 1:4 and 1:6. Preliminary permeability measurements established that the membrane with the ODA-PEO ratio (w:w) 1:2, which exhibited permeabilities of 23.87 and 0.39 Barrers, for CO2 and N-2 respectively, showed the highest ideal selectivity of CO2 over N-2 (61.20) when compared to the other members of the series. The thermal treatment of this membrane was carried out in an oven heated at 200 degrees C, 250 degrees C and 300 degrees C. The heating at 200 degrees C caused a large increase in N2 and CO2 permeability coefficients (1.05 and 57.25 Barrers respectively) which resulted in a slight decrease of the selectivity. It should be mentioned that while the permeability of CO2 increased considerably (similar to 300%), the selectivity just suffered a small drop (similar to 10%), and hence, the productivity of this membrane is much better after the thermal treatment. Thermal treatment of the membrane above 200 degrees C produced successive increments on the permeability coefficients of both gases, while ideal selectivity was maintained.