The phase behavior of poly (N-isopropylacrylamide) (PNIPAAm) in solutions composed of water and methanol was studied at 25 degrees C. The pair of solvents used to dissolve PNIPAAm has been selected for the purpose to perform a cononsolvent system. From the observed phase behavior, PNIPAAm was soluble in either water or methanol individually but liquid-liquid demixing was observed in water/methanol mixtures. Flory-Huggins formalism including three binary interaction parameters and one ternary interaction parameter was used to analyze the phase behavior of the cononsolvent system. The mechanism of cononsolvency and its relation with the ternary interaction parameter were discussed. In addition, the use of two solvents serving as a cononsolvent system, replacing the traditional solvent-nonsol vent pair, for the membrane formation was investigated. Regardless of water or methanol being used as the solvent, it showed a rapidly precipitating system and macrovoid morphology due to liquid-liquid demixing was obtained. Trend 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 non sot vent-solvent-polymer can be extended to a cononsolvent-polymer system. (c) 2005 Elsevier Ltd. All rights reserved.