Thermal stability studies of electrolytes with mixtures of LiPF6 and LiBF4 were carried out using differential scanning calorimetry. The solvent was a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in the volume ratio of 3:3:1, respectively. We expected the occurrence of two independent exothermic peaks associated with LiPF6 at the lower temperature and lithium LiBF4 at the higher temperature, due to decomposition reactions resulting in the Lewis acids PF5 and BF3. Instead, the mixed salt electrolyte exhibited a single exothermic peak. We deduced that the HF produced by the reaction of LiPF6 with solvent was the reason for the existence of one exothermic reaction peak. The HF may react with LiBF4 to give HBF4, which is very unstable and decomposes easily to HF and BF3 at a lower temperature than the decomposition temperature of LiBF4 itself. By comparison, a thermal study of a mixed salt electrolyte including LiPF6 and LiN(SO2CF3)(2) showed that the exothermic reaction of LiN(SO2CF3)(2) with solvents is also influenced by HF produced in the reaction of LiPF6 with solvents but that the strength of the influence is small compared with its effect on an electrolyte mixture including LiBF4. (C) 2004 The Electrochemical Society.