In this study, nylon 6 membranes were prepared in a water coagulation bath with two types of solvents, CaCl2methanol (CaClMe) and formic acid (FA). The morphology of the membranes, which was controlled by the phase behavior of their solutions, were connected to the mechanism of demixing, including liquidliquid and liquid-crystallization. Ternary phase diagrams showed that the CaClMe system coagulated significantly faster than the FA system. As observed by scanning electron microscopy, the CaClMe membrane had a porous, interconnected pore structure with macrovoids, whereas the FA membrane had a dense, spherulitic surface with a closed cell morphology. The high reaction surface of the CaClMe membrane with dye molecules provided outstanding dye rejection. Also, thermal analysis by differential scanning calorimetry showed that the slow coagulation of the FA system facilitated the formation of stable -form crystals rather than a metastable -form structure. The results show that the phase-separation mechanism was switched from liquidliquid to liquid-crystallization through a change in the solvent type from CaClMe to FA. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013