A novel cross-linking approach for a carboxylic acid-containing 6FDA-based copolyimide is presented and characterized. This new type of cross-linking renders the polyimide insoluble in typical solvents and greatly enhances the resistance to plasticization under high CO2 pressure. Charge transfer complexing, oligomer cross-linking, decomposition, and dianhydride formation are all ruled out as possible causes of these enhanced properties. Rather, high thermal annealing temperatures decarboxylate the pendant acid group which creates a phenyl radical capable of attacking other portions of the polyimide for cross-linking. CO2 permeation isotherms reveal the enhanced stability against plasticization, and IR and C NMR confirm the evolution Of CO2 and loss of the carbonyl carbon from the polymer as a result of the cross-linking.