The formation of phase inversion membranes made by immersion precipitation from a ternary system consisting of an amorphous polymer, a solvent, and a nonsolvent has been rationalized by assuming a gelation process. The treatise is based on a hypothesis developed by Berghmans for a binary system consisting of a polymer and a solvent. The ternary phase diagram for the mixture of H2O-NMP (N-methylpyrrolidone)-PES (poly(ether sulfone)) was determined at 25 degrees C. In this system three boundaries could be identified, i.e., a vitrification, a gelation, and a binodal boundary. It was found that for this system and a system composed of H2O-DMAC (N,N-dimethylacetamide)-PSF (polysulfone), vitrification is the only mechanism responsible for the structure fixation of membranes prepared by liquid-liquid demixing due to immersion precipitation. For the system of MPD (2-methyl-2,4-pentanediol)NMP-PES, it was found that a membrane can be formed by vitrification as a result of immersion precipitation prior to liquid-liquid demixing. The depressions of the glass transition temperature of PES by the addition of FP (N-formylpiperidine), DMSO (dimethyl sulfoxide), DMAC, or NMP as well as by a mixture of NMP and water are determined.