The gas permeation stability of an asymmetric polyimide membrane with a thin and defect-free skin layer has been investigated. The polyimide used in this study was synthesized from the fluorinated dianhydride (6FDA) and the aromatic diamine not containing a CF3 group. The asymmetric polyimide membranes were prepared by a dry/wet phase inversion, and the apparent skin layer thickness of the asymmetric membrane was 55 nm. The gas permeances of O-2, N-2, CH4, and CO2 through the asymmetric membranes at 35 degreesC and at pressures up to 760 cmHg have been determined using a high vacuum apparatus. We specifically focused on the CO2 permeation stability of the asymmetric polyimide membrane with a 55 nm skin layer exposured to a CO2 pressure of 760 cmHg. The CO2 permeances of the asymmetric membrane showed almost constant values during the repeated permeation experiments. Additionally, the C-13 T-1 values of the asymmetric membrane repeatedly measured at 760 cmHg were quite similar to those for the original asymmetric membrane before the gas permeation experiment. These results indicate that the plasticization of the asymmetric polyimide membrane was not caused by CO2. We postulated that the absence of bulky CF3 groups on the aromatic diamine inhibited plasticization of the polyimide by CO2.