Poly(vinylidene fluoride) microporous membranes were prepared via the thermally induced phase separation process using diluent mixture of dibutyl phthalate (DBP) and di(2-ehylhexyl) phthalate (DEHP). The effects of the DBP ratio to DEHP in the diluent mixture on the phase diagrams were investigated. The phase diagrams can be controlled successfully by varying the DBP ratio in the diluent mixture. The compatibility between polymer and diluent mixture was characterized to explain the features in the phase diagram. The cross-sectional structures of membranes were investigated and related to the DBP ratio in diluent mixtures, initial polymer concentration, and cooling rate. When the DBP ratio to DEHP was 30 wt/70 wt, only spherulites structure was obtained at the polymer concentration of 30 wt % and at the cooling rate of IO'C/min, whereas the cellular structure was detected as the polymer concentration increased or the cooling rate deceased. In the case of the systems with only S-L phase separation, only spherulites structures were observed and the sizes of spherulites increased with increase in the polymer concentration and with decrease in the cooling rate. (c) 2007 Wiley Periodicals, Inc.