6FDA based polyimides possess outstanding properties, with a high free volume fraction entailing in high permselectivity properties for gas separation processes. However, they are very sensitive to aging process. Copolyimdes 6FDA DAM:DABA were synthesized using different ratios of diamines: (9:1), (3:1) and (2:1) and the homopolyimide 6FDA DAM. Thin Film Composite membranes were prepared and thermally annealed at 300 degrees C for 1 h to improve CO2 induced plasticization resistance. For exploring the future applications of these polymers, long term performance studies of TFC membranes are absolutely needed. For this purpose, TFC membranes were tested for 180 h at 35 degrees C using a 50/50 vol./vol. CO2/CH4 feed composition at 8 bars of transmembrane pressure and aging was done under similar conditions. As a result, a more accurate behavior of long term performance under working conditions can be disclosed. Overall, the drop of performance of all TFC membranes was minor, if it is compared with other studies of aging under ambient conditions. It is explained because polymeric materials experience certain swelling under continuous working conditions due to the presence CO2 and they cannot rearrange themselves as fast as under ambient conditions. 6FDA based polyimides are optimal candidates for industrial applications without losing their properties over extended period of time.