Membranes of poly(vinylidene fluoride) copolymer with hexafluoropropylene (PVDF-HFP) have been prepared using a supercritical fluid-phase separation process in which CO2 acts as the nonsolvent. A series of experiments have been performed at various polymer concentrations, temperatures, and pressures using acetone as the liquid solvent. When the polymer concentration ranged between 1 and 20% (w/w), we obtained membranes characterized by cellular structure with mean cell diameters ranging from 2 to 6 Am or by bicontinuous structures formed by a sub-micrometric porous network. In both cases, a leafy-like submorphology has been observed. The effect of pressure (from 80 to 200 bar) and temperature (from 35 to 55 degrees C) on the membrane formation process has been studied; in particular, upon increasing the pressure and decreasing the temperature, the membrane structure changed from cellular to bicontinuous. Membrane formation mechanisms have been proposed, considering a competition between liquid-liquid demixing, that leads to the formation of cellular or bicontinuous structures, and solid-liquid demixing, that causes the formation of the leafy-like morphology.