The phase behavior of poly(ether imide) (PEI) in solutions composed of N-methyl-2-pyrrolidone (NMP) and methylene chloride (MC) was studied at 25degreesC. The pair of solvents used to dissolve PEI has been selected for the purpose to perform a cononsolvent system. From the observed phase behavior, PEI was soluble in either NMP or MC individually but liquid-liquid demixing was observed in mixtures of NMP and MC. However, no cononsolvency was found by the theoretical prediction on the basis of Flory-Huggins formalism including three binary interaction parameters. Therefore, attempts were made to correlate the phase behavior of a cononsolvent system with the modified Flory-Huggins theory using a ternary interaction parameter. A good prediction was obtained when a composition-dependent ternary interaction parameter was included into calculations. In addition, the mechanism of cononsolvency and its relation with the ternary interaction parameter in the cononsolvent systems were discussed. Based on the analysis of IR spectroscopy, the ternary interaction parameter correlates well with a more intermolecular complexation of NMP with MC in the presence of PEI. Thus, the driving force for cononsolvency results from the formation of the NMP-MC complexes favoring over NMP-MC-PEI contacts, leading to exclude PEI segments in the vicinity of the NMP-MC complexes. (C) 2003 Elsevier Science Ltd. All rights reserved.