Polysulfone(PSF) membranes were prepared by coagulating a PSF solution film with or without addition of water. when a solution film was solidified by evaporation of solvent, polymer molecules in the film were precipitated by vitrification. With addition of water, the cast solution was demixed by the liquid-liquid phase separation due to the nucleation of polymer-lean phase. The growth of the separated two phases continued until the polymer-rich phase vitrified. The polymer-rich phase could be demixed again before precipitation. Solid membranes for measuring the nucleotides were prepared from a casting solution including GAYS, an inorganic fluor. In the membrane solidified via vitrification by the solvent evaporation, GAYS particles were dispersed in the solidified polymer matrix. On the contrary, when the solidification process involved the liquid-liquid phase separation due to the nucleation of the polymer-lean phase, GAYS were separated from the polymer matrix and retained in the polymer-lean phase.