Thermal stability of cathode materials, including LiCoO2, LiNiO2, LiMn2O4, V2O5, V6O13, and Li x MnO2 in contact with poly(ethylene oxide) (PEO) based solid polymer electrolyte was systemically investigated by means of thermal analysis in combination with X-ray diffraction technique (XRD). The differential scanning calorimetry (DSC) analysis showed significant exothermic reaction of both LiNiO2 and LiCoO2 in contact with the polymer electrolyte. LiMn2O4 was less reactive compared with LiNiO2 and LiCoO2. V2O5, V6O13, and Li x MnO2 were also found less reactive, especially in their discharge states. The XRD results indicated that the thermal decomposition products of the cathode material were the low valance metal oxides, suggesting the exothermic reaction was an oxidation reaction of the polymer electrolyte with active material. The decomposition temperature is somehow dependent on the potential of the cathode active materials. Cyclic voltammetry reveals that PEO based solid polymer electrolyte is stable up to 5.0V versus Li/Li+ at a blocking electrode, whereas it decomposes at ca 3.8V when contacted with a carbon composite electrode.