The effect of NH3-plasma treatment on glassy polyethersulfone (PES) membranes upon the diffusion process for CO2, O-2, and N-2. was investigated from the permeability measurements. The permeation behavior for O-2 and CO2 in untreated and NH3-plasma-treated PES membranes was simulated well in terms of the dual-mode mobility model. For O-2 transport, NH3-plasma treatment on PES membrane had a little influence on the diffusion process of Langmuir species and very little influence on the diffusion process of Henry's law species. For CO2 transport, it promoted the transport of Henry's law mode but had very little influence on the transport process of Langmuir species. Both the mean permeability coefficient to CO2 and the ideal separation factor for CO2 relative to N-2 took maximum values at a treatment power of 40 W. (C) 2004 Wiley Periodicals, Inc.