A phase diagram of poly(methyl methacrylate) in mixtures of water and 2-propanol, individually nonsolvents for the polymer, was studied at 25 degrees C. For this system, there were two liquid-liquid demixing regions separated by a miscible region. This cosolvent phenomenon was thought to be a joint effect of the nonsolvents. The phase behavior was modeled according to modified Flory-Huggins chemical-potential equations, which accounted for the possible contribution from a ternary interaction in terms of a lumped parameter, chi(123). The calculated phase-equilibrium curves (binodals) agreed well with the measured results. By contrast, if only binary interaction parameters were considered, computations yielded binodals whose compositions departed significantly from the measured data. Using the wet phase inversion method with casting dopes selected on the basis of the phase diagram, are prepared membranes with microporous structures in various coagulation baths,