The preparation of membranes of nylon 4,6 by diffusion-induced phase separation (DIPS) using formic acid as a solvent and water as a nonsolvent was studied. Nylon 4,6 is a semi-crystalline polymer; phase separation from a solution can occur by solid-liquid (s-l) de-mixing as well as by liquid-liquid (l-l) demixing. Upon quenching films of solutions with low polymer concentration (< 17 wt %) in a nonsolvent bath containing water, the morphology of the membranes show a foam-like structure typical for l-l demixing. When phase separation is induced by water vapor a transition in structure occurs from the cellular type to a morphology typical for s-l phase separated films. At higher polymer concentrations membranes exhibit structures consisting of spheres or smaller crystal-like units resulting from an s-l phase separation process. The addition of 2 wt % or more of water to polymer solutions with low concentration (up to 15 wt %) resulted in s-l demixing as well. In a DIPS process s-l demixing is kinetically competitive with l-l demixing if nuclei are already present in the starting solutions (heterogeneous nucleation), or if a relatively long time is available for crystal nuclei to be formed. The morphology resulting from s-l demixing is a result of spherulitic crystallization. A certain concentration of nuclei or of precursor particles already present results in a small nucleation density during precipitation and thus large spherulites can be grown; at higher polymer and/or water concentrations the nucleation density increases resulting in an axialitic morphology of the membranes.