Polymer blend (poly(ethylene oxide) (PEO)poly(propylene oxide) (PPO)) systems with two different mole ratios, complexed with LiCF3SO3 salt, have been characterized at various temperatures and compositions using a thermo-optical analysis (TOA) technique. We also developed a new melting point depression theory based on the modified perturbed hard sphere chain model to interpret phase behavior of polymer blend electrolyte systems. The obtained results show that the eutectic points move toward higher T-m and lower weight fraction of salt with decreasing PEO mole ratio and also indicate that the mole ratio of PEO-PPO for each polymer blend plays an important role in determining the eutectic point of the polymer blend system. (c) 2005 Wiley Periodicals, Inc.