A novel polyetherimide (PEI)/benzophenone/triethylene glycol ternary system with upper critical solution temperature was developed to prepare PEI microporous membranes via thermally induced phase separation approach. In contrast to most polymer/diluent systems, this system was phase-separated into a PEI/benzophenone polymer rich phase and a triethylene glycol/benzophenone diluent rich phase upon cooling from one-phase region. The distribution coefficient of benzophenone in the two phases was estimated to be 1.23. The remaining benzophenone in the PEI rich phase resulted in the decrease of solidification temperature with the decrease of PEI concentration in the ternary system. In the PEI membranes prepared from this system, uniformly distributed microsized pores from the liquid-liquid phase separation were observed, and the pore diameters were found to be reduced with the increase of PEI concentration and the increase of cooling rate. (C) 2010 Elsevier B.V. All rights reserved.