Abstract: A modified free - volume diffusion model for binary aqueous mixtures in pervapo-ration was developed on the basis of free -volume diffusion theory. Two coupling coefficients were introduced to describe the coupling effect of diffusion for nonideal system in pervapora-tion membrane. Combined with the membrane phase concentrations which were calculated from experimental adsorption data of four aqueous solutions in polyimide (PMDA -ODA) membrane, the experimental pervaporation data of the four mixtures were correlated with this model, good agreement was obtained between the calculated and experimental values.