Crystalline poly(ethylene-co-vinyl alcohol) (EVAL) membranes from the water-2-propanol cosolvent were prepared by immersion-precipitation at 60 degrees C. The use of two nonsolvents serving as a cosolvent system, replacing the traditional solvent-nonsolvent pair, for the membrane formation was investigated. The two-nonsolvent proportion in which they are mixed to produce the cosolvent system and the prediction of the membrane structures were based on the complex ternary phase diagram at 60 degrees C. It was attempted to relate the influence of liquid-liquid demixing and solid-liquid demixing on the membrane morphologies to the ternary phase diagram. Two different morphologies and two different demixing rates could be obtained by using water and 2-propanol, respectively, to precipitate the cosolvent system. When water was used as the precipitation medium, it showed a rapidly precipitating system and cellular morphologies were obtained due to liquid-liquid demixing. In contrast, when 2-propanol was used as the precipitation medium, it showed delayed demixing and crystallization-induced particulate morphologies were formed. Trends expected on the basis of the phase diagram was in reasonable agreement with the observed membrane morphology. Therefore, the principles of membrane formation established for the ternary systems with nonsolvent-solvent-polymer can be extended to a nonsolvent-nonsolvent-polymer system.