Thermal analysis of poly(ethylene oxide) (PEO) with three different salts, MnBr2, MnI2 and LiCF(3)O(3)3 has been carried out at various temperatures and compositions. Optical microscopy and differential scanning calorimetry have been used to collect data necessary for the phase diagrams. The melting point depression phenomena are observed to be explicable in terms of thermodynamic equilibrium of the amorphous phase with the crystalline phase. To this end, a thermodynamic model appropriate to describe the crystalline-amorphous phase in the semicrystalline polymer is developed based on an extended Flory-Huggins equation. Quantitative description according to the proposed model is in good agreement with the experimentally observed transition temperature of a given system including an eutectic point.