We studied the poly(ethylene oxide)-LiCF3SO3 system in order to determine the stability of the polymer electrolyte when applied to various electrode pairs. We investigated the thermal stability of the polymer electrolyte (PEO)8LiCF3SO3 in contact with lithium metal and the cathode materials, amorphous V2O5, LiV3O8, V6O13, and LiCoO2. The properties of the electrolyte were measured by complex impedance analysis and TG-DTA. It was demonstrated that the electrolyte was stable with lithium metal up to 220-degrees-C, and also was stable with the above mentioned cathode materials at temperatures below about 160-degrees-C. The decomposition rate of the polymer electrolyte was faster in air than in Ar gas.