Liquid-liquid phase separation phenomena are investigated for a quaternary system containing two polymers, a solvent, and a nonsolvent for one of the polymers which also is a solvent for the other polymer. The phase separation behavior studied is related to the membrane-forming properties of a system containing a macromolecular additive as a second polymer. To visualize the parts of the three-dimensional quaternary phase diagrams, semiternary cross-sections are used in which two components are regarded as a "lumped’ component. Cloud point and shadow curves are given. The critical point valid for a ternary system is extended into a critical curve. It is found that the critical curve at larger molecular weights of the second polymer is situated at higher concentrations of the membrane-forming polymer. A high molecular weight of this second polymer causes the phase diagram to become insensitive to the various interaction parameters. At constant molecular weight of the second polymer, the critical curve shifts to higher polymer concentrations upon increasing the concentration of the second polymer. Interaction effects appear to have a marginal influence, as long as the component pairs that were assumed to be miscible remain miscible.