Microporous cellulose acetate membranes were prepared by a thermally induced phase separation (TIPS) process. Two kinds of cellulose acetate with acetyl content of 51 and 55 mol % and two kinds of diluents, such as 2-methyl-2,4-pentandiol and 2-ethyl-1,3-hexanediol, were used. In all polymer-diluent systems, cloud points were observed, which indicated that liquid-liquid phase separation occurred during the TIPS process. The growth of droplets formed after the phase separation was followed using three cooling conditions. The obtained pore structure was isotropic, that is, the pore size did not vary across the membrane. In addition, no macrovoids were formed. These pore structures were in contrast with those usually obtained by the immersion precipitation method. (C) 2003 Wiley Periodicals, Inc.