Modified Flory-Huggins theory with concentration dependent binary interaction parameters is often used to predict the phase diagram in a ternary nonsolvent/ solvent/polymer system. The interaction parameters in this model are usually determined experimentally which limit the model predictive capability. In this paper a compressible regular solution (CRS) model was used to predict the phase diagram of a membrane forming water/tetrahydrofuran/poly(vinyl chloride) system, by pure component properties such as, solubility parameter, coefficient of thermal expansion and hard-core volume. In this respect, the binodal curve, spinodal curve and the critical point were determined by numerical calculations. Properties of components that were needed for these calculations have been taken from available data in the literature. Experimental cloud point data were obtained by the titration method of PVC in THF solutions with concentrations reaching 15 wt%. The good agreement between the theoretical binodal and experimental cloud points indicates that this model is a promising method to calculate the theoretical phase diagram for membrane forming systems, with particular attention to the fact that no adjustable parameters such as binary interaction parameters should be used for theoretical calculations. The result of these calculations revealed that small amounts of water (less than 10%) is needed for liquid-liquid phase separation in water/tetrahydrofuran/poly(vinyl chloride) system.

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