Polysulfone membranes have been prepared from polymer/N-methylpyrrolidone (NMP) and polymer/chloroform solutions using a supercritical fluid-phase inversion process in which CO2 acts as the non-solvent. Series of experiments were performed at various polymer concentrations, temperatures and pressures. We operated at polymer concentrations ranging between 15 and 30% (w/w) in NMP and between 2 and 10% in chloroform obtaining membranes with a cellular structure and different cell dimensions. On increasing the percentage of polymer in the solution, the mean diameter of the cells varied from about 3 to 20 mu m as polymer concentrations varied from 30 to 15% (w/w) in NMP and from about 2 to 4 mu m, for polymer concentrations from 10 to 2% (w/w) in chloroform. The membrane formation pressures have been varied between 100 and 200 bar and temperatures between 35 and 65 degrees C for both solvents. We observed that on increasing the pressure and decreasing the temperature, the cell size decreased. These results can be related to the thermodynamics of the polysulfone membranes formation process. (c) 2005 Elsevier B.V. All rights reserved.