The phase separation behavior of a poly(ether imide) (PEI)/NN-dimethyl acetamide (DMAc)/nonsolvent system was investigated. Three kinds of nonsolvents were used in the study: H2O, ethanol, and acetic acid (AA). It was found that the three systems (PEI/DMAc/H2O, PEI/ DMAc/ethanol, and PEI/DMAc/AA) agree with the linearized cloud point (LCP) relation. The binodal lines of the three systems were calculated according to the LCP relation. The binodal line of the PEI/DMAc/(H2O + DMAc) system was also calculated according to the LCP relation of the PEI/DMAc/H2O system. The phase separation of the PEI/ DMAc/(H2O + AA) system was studied, and the results agree with the LCP relation. These results can offer useful information for the establishment of dope and coagulation media using for the fabrication of a PEI membrane. (C) 2003 Wiley Periodicals, Inc.