The thermal stability of some mixed-solvent electrolytes used in lithium cells was measured by differential scanning calorimetry (DSC) using airtight containers. The electrolytes used were ethylene carbonate ( EC)+diethylcarbonate (DEC), EC+dimethylcarbonate (DMC), propylene carbonate (PC)+DEC, and PC+DMC in which was dissolved 1M LiPF6 or 1M LiClO4. The influence of water addition and lithium metal addition on the thermal behavior of these electrolytes was also investigated. The exothermic peak of LiPF6 electrolytes containing DEC was found at 255°C, and the peak temperature of the electrolytes containing DEC was 15–20°C lower than that of LiPF6 electrolytes containing DMC. This effect was also observed in the electrolytes including LiClO4. DMC was found to be more reactive than DEC. The thermal behavior of various kinds of LiPF6 electrolytes with lithium metal was measured by DSC. The exothermic reaction of 1M LiPF6/EC+DEC, 1M LiPF6/EC+DMC, and 1M LiPF6/PC+DMC with lithium metal began at the melting point of lithium metal because of the break down of the solid electrolyte interface (SEI). The temperature was approximately 180°C, whereas the self-heating of 1M LiPF6/PC+DEC occurred before the melting point of lithium metal. The temperature at which the self-exothermal reaction began was 140°C. Therefore, the lithium metal in this electrolyte was found to be thermally unstable. When water was added to the above electrolytes with lithium metal, the exothermic reaction began at less than 130°C, probably due to a collapse of SEI in response to the HF that was a product of the reaction between LiPF6 and the added water.