Microporous poly(vinylidene fluoride-co-hexafluoropropylene) membranes were prepared by phase separation with supercritical CO2 as non-solvent. The dry flat membranes were directly obtained after CO2 pressure was diminished without post-treatment. Effects of experimental conditions such as the CO2 pressure, the polymer concentration and the temperature on the membrane porosity and average pore size were investigated. It was found that these membranes have a "sponge-like" and asymmetric structure. The average pore size increases with increasing CO,) pressure and temperature, but decreases with increasing polymer concentration. The porosity can reach approximately 88% and the average pore size was 5 mu m or so. The crystallinity and thermal stability of membranes were characterized by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results showed that the membranes are a crystals with excellent thermal stability. (c) 2005 Elsevier B.V. All rights reserved.