It is known that CO2 acts as a plasticizer in CO2/CO4 membrane separations at elevated pressures. The polymer matrix swells upon sorption of CO2, accelerating the permeation of CH4. As a consequence, the polymer membrane loses its selectivity. To overcome this effect, plasticization should be minimized. We succeeded in stabilizing the polymer membrane by a thermal treatment. For this purpose the polyimide Matrimid 5218 is used as model polymer. In single gas experiments with CO2, the untreated membrane normally shows a minimum in its pressure dependence on permeability, whereas the treated membranes do not. Membrane performances for CO2/CO4 gas mixtures showed that the plasticizing effect indeed accelerates the permeation of methane. The heat treatment clearly suppresses this undesired methane acceleration Additionally to the pure and mixed gas permeation results, process calculations reveal valuable information as to what extent the stabilized membranes show improved membrane performance. The favourable performance of the stabilized membrane can be attributed to less methane loss and therefore a higher recovery, resulting in higher profit from gas sales.