Decarboxylation-induced thermal cross-linking occurs at elevated temperatures (similar to 15 degrees C above glass transition temperature) for 6FDA-DAM:DABA polyimides, which can stabilize membranes against swelling and plasticization in aggressive feed streams. Despite this advantage, such a high temperature might result in collapse of substructure and transition layers in the asymmetric structure of a hollow fibers based on such a material. In this work, the thermal cross-linking of the 6FDA-DAM:DABA at temperatures much below the glass transition temperature (similar to 387 degrees C by DSC) was demonstrated. This sub-T-g cross-linking capability enables extension to asymmetric structures useful for large scale membranes. The resulting polymer membranes were characterized by swelling in known solvents for the uncross-linked materials, TGA analysis, and permeation tests of aggressive gas feed stream at higher pressure. The annealing temperature and time clearly influence the degree of cross-linking of the membranes, and results in a slight difference in selectivity for membranes under various cross-linking conditions. Results indicate that the sub-Tg thermal cross-linking of 6FDA-DAM: DABA dense film membrane can be carried out completely even at a temperature as low as 330 C. Permeabilities were tested for the polyimide membranes using both pure gases (He, O-2, N-2, CH4, CO2) and mixed gases (CO2/CH4). The selectivity of the crosslinked membrane can be maintained even under very aggressive CO2 operating conditions that are not possible without cross-linking. Moreover, the plasticization resistance was demonstrated up to 700 psia for pure CO2 gas or 1000 psia for SO% CO2 mixed gas feeds.