Phase separation rate during porous membrane formation by immersion precipitation was investigated by light scattering in a polyimide/N-Methylpyrrolidone (NMP)/water system. In the light scattering measurement, plots of scattered intensity against scattered angle showed maxima in all cases, which indicated that phase separation occurred by a spinodal decomposition (SD). Characteristic properties of the early stage of SD, such as an apparent diffusion coefficient D and an interphase periodic distance Lambda, were obtained. ie growth process of Lambda was also followed by light scattering. The growth rate had the same tendency as D-app when water content in the nonsolvent bath and the polymer concentration in the cast solution were changed. The pore size of the final membrane increased with decreasing water content, which was opposite to the tendency of A growth rate. (C) 2003 Wiley Periodicals, Inc.