Cast-leaching experiments were carried out to investigate the dynamics of membrane formation by immersion precipitation, with an emphasis on the outflow of the solvent from casting solutions during the phase-separation process. The casting solutions, consisting of poly(ether sulfone) as the polymer, N-methyl-2-pyrolidone as the solvent, and water (H2O), isopropyl alcohol, 1-butanol, and diethylene glycol as nonsolvent additives (NSAs), were immersed in a coagulation bath. Two thermodynamically vastly different coagulants - H2O, a strong coagulant, and ethylene glycol, a weak coagulant - were used to study the effect of the coagulant on the dynamics of membrane formation. The results showed that the outflow of the solvent during the initial stage of membrane formation was controlled by Fickian diffusion within the extremely wide range of conditions studied, that is, polymer concentrations of 10-38%, approaching ratios of 0-0.95, and thermodynamically vastly different NSAs and coagulants. The role of the initial state of the membrane-forming solution, especially the conformational state of macromolecules in the membrane-forming process, was examined. In contrast to those works on the behavior of small molecules, an attempt was made to qualitatively interpret membrane formation from the viewpoint of macromolecules. (C) 2005 Wiley Periodicals, Inc.