The separation of water/ethanol mixtures was investigated by poly(acrylonitrile-co-acrylic acid) and by poly(acrylonitrile-co-acrylic acid)/poly(ethylene oxide) blend membranes. The flux increased with the content of acrylic acid in copolymers and the selectivity remained constant. The marked increase of the selectivity was observed for blend membranes of a certain blend ratio, suggesting that the two polymers are partially miscible. Poly (ethylene oxide) in blends was thought to act as a plasticizer as well as a preferentially water absorbing and diffusing component.