Solid polymer electrolytes based on poly(ethylene oxide) (PEO) and lithium trifluoroacetate (CF3COOLi) were prepared by a solvent casting method by different concentrations of salt. The membranes resulting of slow evaporation were analyzed by differential scanning calorimetry (DSC) and its phase diagram was drawing for a second thermal scan. The DSC curves on the first thermal cycle show three anomalies at 303, 338 and 393 K, that are slightly shifted with the salt content. For high concentrations, the intensity of the endothermic peak associated with the melting point of PEO (338 K) decreases, that one for the third transition increases with the salt content, the second peak (melting point of PEO) disappear when the concentration reaches the value of X = 0,405 (given in fraction of mass: salt/(salt + polymer)). These results and the change of conductivity demonstrate the formation of a new complex between the PEO and the CF3COOLi.