AbstractMelting‐point depression in miscible polymer blends is interpreted with Flory‐Prigogine's equation‐of‐state theory (FP theory) and Sanchez‐Lacombe's lattice fluid theory (LF theory). The equations for equilibrium melting point depression in polymer mixtures are proposed from both the FP and LF theories. For miscible poly(ether‐sulfone) (PES)/poly(ethylene oxide) (PEO) blends, the proposed equations are tested. The interaction parameters,X12in FP theory and ζ12in LF theory, can be determined with these equations. The theoretically predicted equilibrium melting‐point depression is subdivided into three terms, namely, the equation‐of‐state, the entropy and the contact interaction terms. When the estimated interaction parameters are converted to the heat of mixing by use of both theories, the composition dependence of the heat of mixing can be properly predicted. Using the interaction parameters obtained from the melting‐point depression in PES/PEO blends, the spinodal curves are simulated from both th