We establish a new thermodynamic framework to describe the electrochemical properties of solid polymer electrolyte/ salt systems. The Pitzer-Debye-Huckel expression is introduced to the modified double lattice-nonrandom model to take into account long-range electrostatic forces between molecules. After developing full descriptions for the electrochemical properties of a poly( ethylene oxide) (PEO)/NaCF3SO3 system, the open-circuit voltage (OCV), diffusion coefficient, and ionic conductivity are calculated and compared with experimental data. We predict OCVs and diffusion coefficients of PEO/ZnBr2 and PEO/ZnI2 systems with the parameters obtained from phase diagrams and ionic conductivity data. The quantitative description according to the proposed model is in good agreement with the experimental data. (c) 2005 The Electrochemical Society.