Ion beam irradiation is an easily controlled method to modify the chemical structure and microstructure of polymers including the fractional free volume, free volume distribution and chain mobility, thus altering the gas transport properties of the irradiated polymers. The previous paper focused on the impact of H+ ion beam irradiation on chemical structural evolution of the polyimide Matrimid((R)). This paper focuses on the impact of H+ ion beam irradiation on microstructure and gas permeation properties of Matrimid((R)). Irradiation at low ion fluence resulted in slight decreases in permeabilities for five gases (i.e., He, CO2, O-2, N-2, and CH4) with increases in permselectivities for some gas pairs (e.g., He/CH4 and He/N-2). In contrast, irradiation at relatively high ion fluences resulted in simultaneous increases in permeabilities and permselectivities for most gas pairs (e.g., He/CH4, He/N-2, O-2/N-2, and CO2/CH4). While Matrimid((R)) has bulk gas permeation properties that are below the range of commercially interesting polymers, samples irradiated at high ion fluences exhibited significant improvement in gas separation performances. (c) 2006 Wiley Periodicals, Inc.