The thermal stability of mixed-solvent electrolytes used in lithium cells was investigated by differential scanning calorimetry (DSC) through the use of airtight containers. The electrolytes used were propylene carbonate (PC) and ethylene carbonate (EC)+PC, in which was dissolved 1M LiPF6, 1M LiBF4, 1M LiClO4, 1M LiSO3CF3, 1M LiN(SO2CF3)2, or 1.23M LiN(SO2CF3)(SO2C4F9). The influence of lithium metal or the Li0.5CoO2 addition on the thermal behavior of these electrolytes was also investigated. The peak temperature of PC-based electrolytes increased following the order of LiPF6<LiClO4<LiBF4<LiN(SO2CF3)2<LiSO3CF3<LiN(SO2CF3)(SO2C4F9). The order of peak temperature of EC–PC-based electrolytes shows a similar tendency to that of EC–PC-based electrolytes, with the exception of the LiN(SO2CF3)2 electrolyte. The EC–PC-based electrolytes with Li metal show a more stable profile compared with the DSC curves of the PC-based electrolytes with the Li metal. The solid electrolyte interphase (SEI) covers the surface of the Li metal and prevents further reduction of the electrolytes. EC may form a better SEI compared with PC. The PC-based electrolytes of LiSO3CF3, LiN(SO2CF3)2 and LiN(SO2CF3)(SO2C4F9) with the coexistence of Li0.49CoO2 show a broad peak at around 200°C, which may be caused by the reaction of the Li0.49CoO2 surface and electrolytes. The PC-based electrolytes of LiPF6, LiClO4 and LiBF4 with Li0.49CoO2 show exothermic peaks at higher temperatures than 230°C. The peak temperatures of the EC–PC-based electrolytes with the coexistence of Li0.49CoO2 are nearly the same temperature as the EC–PC-based electrolytes.