Microporous poly(vinyl butyral) (PVB) membranes were successfully prepared by supercritical CO2-induced phase separation. The obtained membrane structures were characterized by scanning electron microscope. The results indicated that the process parameters, such as PVB concentration in casting solution, CO2 pressure, temperature and depressurization time, had significant effects on the membrane structure. The increase of PVB concentration led to a decrease of the mean pore size. With the increase of temperature and CO2 pressure, the top surface of the membrane became dense, the pore size of cross-section increased and the asymmetry became more pronounced. With the increase of the depressurization time, the top Surface became Porous and the pore size of cross-section increased. Macrovoids occurred at a certain distance from the skin layer of the membrane prepared under the pressure of 15 MPa or at the PVB concentration of 19% (w/w). The crystallinity and thermal stability of membranes were characterized by wide-angle X-ray diffraction, differential scanning calorimeter and thermogravimetric analysis. The relations between the membrane characteristics and the process parameters were extensively discussed. (C) 2007 Elsevier B.V. All rights reserved.