Poly(ethylene-co-vinyl alcohol) hollow-fiber membranes with a 44 mol % ethylene content were prepared by thermally induced phase separation. A mixture of 1,3-propanediol and glycerol was used as the diluent. The effects of the ratio of 1,3-propanediol to glycerol in the diluent mixture on the phase diagram, membrane structure, and membrane performance were investigated. As the ratio increased, the cloud point shifted to lower temperatures, and the membrane structure changed from a cellular structure due to liquid-liquid phase separation to a particulate structure due to polymer crystallization. Better pore connectivity was obtained in the hollow-fiber membrane when the ratio of 1,3-propanediol to glycerol was 50:50, and the membrane showed about 100 times higher water permeability than the membrane prepared with pure glycerol. For the prepared hollow-fiber membrane, the solute 20 nm in diameter was almost rejected. (C) 2004 Wiley Periodicals, Inc.