The usual two-solvent technique was used to prepare a series of PEO (MW = 5 x 106) electrolytes containing trivalent salts of Eu with compositions between n = 80 and n = 3 (n-O/M). The films have been characterized in detail by Differential Scanning Calorimetry (DSC), scanning electron microscopy energy dispersive X-ray microanalysis (SEM/EDX) and X-ray powder diffraction. The studies have shown that all the films with n 2 12 exhibit an endothermic peak around 65"C, which is associated with the melting of crystalline PEO. For salt-rich films, 3 I n 5 8, this peak was depressed to an onset of approximately 50Β°C. In addition, all the films showed a broader endotherm between the melting of PEO and the onset of decomposition around 250-260Β°C which is attributed to the melting of crystalline PEO/salts complexes. Films with n z 16 showed a smaller endotherm around 6O"C, which was not observed upon reheating, thus suggesting that it results from the eutectic phase of PEO and PEO/salt complex. Furthermore, a stoichiometric complex with unusual properties was observed for n z 3. It is a glassy, transparent and fragile material when no traces of water can be detected.