This paper presents a quick, useful method for obtaining an equilibrium phase diagram (including the melting point depression curve) for a semicrystalline polymer-diluent mixture from cloud point data. Good agreement was obtained between the theoretically predicted and experimentally generated equilibrium crystallization curves in spite of simplifying assumptions. An accurate extrapolation of the coexistence curve to regions in the phase diagram where experimental generation of cloud point data is not possible was also obtained. Flory’s original polymer swelling theory was used to determine the complete phase diagram. In the system studied (isotactic polypropylene in diphenyl ether), the interaction parameter was assumed to be a function of temperature only. Cloud points were generated experimentally, and an interaction parameter was determined at each temperature. A melting point depression curve was calculated from the temperature dependent interaction parameter and compared with experimentally determined equilibrium melting points. the resulting phase diagram was used to interpret the morphology of microporous structures prepared by the thermally induced phase separation mechanism.