Mass transfer during membrane formation by means of phase inversion for a polymeric system with both a solid-liquid and a liquid-liquid equilibrium was studied on the basis of the theory developed by Reuvers and Smolders. During the first moments of immersion in the coagulation bath, the concentrations at the interface between bath and film are governed by the virtual liquid-liquid equilibrium. This equilibrium no longer exists at a larger time scale. The interfacial concentrations as a result of the local liquid-liquid equilibrium during mass transfer are located deeply in the crystallization region or solid-liquid demixing area and after an induction time the solid-liquid phase separation (crystallization) takes place when membranes are formed with an initial polymer concentration of 20% or larger. The calculated initial concentration profiles show a shallow pattern in polymer content for the films with initial concentration of 20 and 25%. From the calculated initial concentration profiles an isotropic morphology in the final membrane can be expected. A steep increase of the polymer concentration at the interface was observed for the more concentrated films correlated with a skinned morphology in the final membrane.