This study describes the dehydration of an ethanol/water azeotrope during evapomeation using polyion complex cross-linked chitosan composite (q-Chito-PEO acid polyion complex/PES composite) membranes, constructed from quaternized chitosan (q-Chito) and poly(ethylene oxydiglycolic acid) (PEO acid) on a porous poly(ether sulfone) (PES) support. Both the q-Chito/PES composite and the q-Chito-PEO acid polyion complex/PES composite membranes showed high water permselectivity for an ethanol/water azeotrope. Both the permeation rate and the water permselectivity of the q-Chito/PES composite membranes were enhanced by increasing, the degree of quaternization of the chitosan molecule because the affinity of the q-Chito/PES composite membranes for water was increased by introducing a quaternized ammonium group into the chitosan molecule. q-Chito-PEO acid polyion complex/PES composite membranes prepared from an equimolar ratio of carboxylate groups in the PEO acid versus quaternized ammonium groups in the q-Chito showed the maximum separation factor for water permselectivity without lowering the permeation rate. With an increasing molecular weight of PEO acid, the separation factor for water permselectivity increased, but the permeation rate almost did not change. The mechanism responsible for the separation of an ethanol/water azeotrope through the q-Chito-PEO acid polyion complex/PES composite membranes was analyzed by the solution-diffusion model. The permeation rate, separation factor for water permselectivity, and evapomeation index of q-Chito-PEO acid 400 polyion complex/PES composite membrane with an equimolar ratio of carboxylate groups in PEO acid 400 and ammonium groups in q-Chito were 3.5 x 10(-1) kg/(m(2) hr), 6300, and 2205, respectively, and very high membrane performance. The separation factor for water permselectivity for aqueous solutions of n-propyl and isopropyl alcohol was also maximized at an equimolar ratio of carboxylate groups and ammonium groups and was greater than that for an ethanol/water azeotrope. The above results were discussed from the viewpoint of the physical and chemical structure of the q-Chito-PEO acid polyion complex/PES composite membranes and the permeants.