Polyurethane membranes were prepared by an immersion precipitation process. The effects of dope concentration, coagulation bath composition, and the chemical structure of the polyurethane on the morphology of the membranes were studied. The degree of contraction was measured by quenching freshly formed polyurethane membranes in Liquid nitrogen. A mechanism for the formation of membrane morphology during immersion precipitation is proposed. (C) 1999 John Wiley & Sons, Inc.