Asymmetric membrane formation has been studied by using an in situ analysis developed with a Micro Raman spectroscopy, which emphasizes kinetic aspects of the phase inversion process. Changes in composition with time was successfully measured on the gelation bath-side as well as inside the precipitated phase for the polymer/solvent/nonsolvent system of polysulfone/1-methyl-2-pyrrolidinone/ethanol. The results shows that resulting relative mass transfer rates of solvent and nonsolvent during the phase inversion process strongly influence the final membrane morphologies. In addition, ternary compositions at which phase separation initiates were explored by analyzing the coagulation front. (C) 2000 John Wiley & Sons, Inc.