We establish a new melting point depression theory based on the modified double lattice (MDL) model. The proposed work takes into account the long-range interactions caused by the Coulombic interaction between ions and the local composition concept to describe phase behaviors of the solid polymer electrolyte/salt system. The ionic contribution is described by the Pitzer-Debye-Huckel (PDH) ion interaction model and the nonrandom factor proposed by Panayiotou et al. Systems studied in this work are binary mixtures of PEO and various salts. Results show that the ionic interaction and nonrandomness effect of the salt distribution play a major role in determining eutectic points of the given systems. When compared with experimental data, quantitative descriptions of the proposed model show better agreement for the given systems than those of the original MDL model. (c) 2004 Elsevier B.V. All rights reserved.