The effect of NH3-plasma treatment on glassy poly(phenylene oxide) (PPO) membranes on the diffusion process for carbon dioxide (CO2), oxygen (O-2), and nitrogen (N-2) was investigated from the permeability measurements. The sorption equilibria and permeation behavior for O-2 and CO2 in untreated PPO membranes were simulated well in terms of the dual-mode sorption and mobility model. For O-2 transport, NH3-plasma treatment on PPO membrane had an influence on the diffusion process of Henry's law species, whereas for CO2 transport, it promoted the transport of Langmuir mode, presumably through an increased Langmuir capacity constant for CO2.