In this paper, well-ordered macroporous membranes were fabricated from random poly(styrene-co-acrylonitrile) using tetrahyorofuran as solvent by the breath figure method. Influencing factors were investigated systematically including the relative humidity of atmosphere, the concentration of polymer solutions and the temperature. The pore size and the patterns were affected by these factors. The mechanism of pattern formation was also discussed. (c) 2005 Elsevier Ltd. All rights reserved.