Microporous cellulose acetate membranes have been prepared from polymer-acetone 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. The structure of the resulting membranes was analysed using scanning electron microscopy. We operated with polymer concentrations ranging between 5 and 40% (w/w) in acetone obtaining different pore dimensions and membrane structures. Increasing the percentage of polymer in the solution, the structure of the membranes changed from beads-like structure to cellular structure. Polymer concentration also influenced the mean diameter of the pores that ranged from 2 to 50 mum for polymer concentrations from 40 to 5% (w/w). We also tested membrane formation pressures between 100 and 200 bar and at temperature between 45 and 65 degreesC. Pressure influences the change in membrane structure from cellular to beads-like, whereas temperature has a minor influence on pore size: both the effects can be partially related to CO2 density. Cellulose acetate membrane formation mechanisms have also been discussed. (C) 2004 Elsevier B.V. All rights reserved.