Cellulose acetate and polysulfone casting solutions were coagulated in water/solvent mixtures with differing solvent content. Precipitation in pure water yielded skinned membranes. Precipitation in water/solvent mixtures with solvent concentration exceeding a certain minimum value (which is different for different systems) resulted in microporous membranes. This phenomenon has been explained in terms of the model description for the formation of asymmetric membranes as adopted in our laboratory. In this model, the skin formation is related to gelation and the formation of the porous substructure to liquid-liquid phase separation. It is made plausible that the addition of solvent to the coagulation bath favours non-solvent inflow and hence liquid—liquid demixing in the precipitating film.