The effect of solvent content in the coagulation bath on the formation of poly(ethylene-co-vinyl alcohol) (EVAL) membranes by isothermal immersion–precipitation in water–dimethylsulfoxide (DMSO)–EVAL system has been investigated. As a homogeneous EVAL dope is immersed in a harsh bath, e.g., water, instant precipitation occurs initiated by the liquid–liquid demixing process. The formed membrane exhibits an asymmetric morphology with extensive finger-like macrovoids, similar to that observed in most amorphous polymeric membranes. On the other hand, if precipitation takes place in a soft bath containing a substantial amount of solvent, crystallization, rather than liquid–liquid demixing, starts to dominate and a skinless, bi-continuous, particulate membrane becomes the precipitated product. The immersion–precipitation process for the present system has been modeled as a ternary mass transfer problem. The calculated diffusion trajectories and concentration profiles illustrate reasonably the membrane morphologies formed in various immersion conditions.