Integrally skinned PVA membranes have been prepared from PVA/NMP/water system by the phase inversion method in 2-propanol bath, and the pervaporation characteristics of these membranes were investigated. With an increase in the NMP content in the polymer solution, the formation of skin layer depended on the NMP content in the cosolvent, and the overall morphology of membranes changed from porous to dense structure. Particularly, PVA membranes showed an asymmetric structure composed of a dense skin layer and a porous sublayer with finger-like type and cellular structure in solvents of 50-60% NMP compositions. The experimental results from the pervaporation separation of MTBE and methanol mixtures were also in good agreement with the morphology for the formation of skin layer. (C) 2003 Wiley Periodicals, Inc.