The process of pervaporation in which two components diffuse through a nonporous polymer membrane was modelled when one of the penetrants can exert a plasticization action to the membrane material. Thereat a phenomenological model was employed for describing the plasticization effect on the diffusivities for penetrants in the membrane. The sorption equilibria and permeation fluxes for aqueous ethanol solutions in a chitosan membrane were measured, and the permeation fluxes for water were compared with those predicted by the proposed model. The con-centration of sorbed water was linear with its weight fraction(x_A) in the feed solution, whereas the permeation flux of wa-ter was affected by the plasticization action of sorbed water to the polymer. This plasticization effect on the diffusion pro-cess can be simulated in terms of the proposed phenomenological model.