In this study, we focused on the effects of the carbon structures on the gas permeability and selectivity of the asymmetric polyimide membranes prepared by He-irradiation using iron (III) acetylacetonate (AAI) as the catalyst. The irradiated asymmetric membranes consisted of a defect-free carbonized ultrathin skin layer and indicated a molecular sieving-like effect due to the surface carbonization similar to carbon molecular membranes. The Raman spectrum of the irradiated membranes included two well-known D (1360cm(-1)) and G (1580cm(-1)) peaks and two additional new peaks around 1150 and 1500 cm(-1). Interestingly, the graphitic and amorphous structures at 1580 and 1500 cm(-1) in the membranes increased with an increase in the AAI content. Especially, it was found that the catalyst had a profound effect on the graphitic and amorphous structures of the ion-irradiated membrane. Finally, we concluded that the amorphous structure in the membrane had an influence on the gas permeability and that the graphitic structure formed a barrier to the gas permeability and had an influence on the gas selectivity. (C) 2011 Elsevier B.V. All rights reserved.