A silicon/disordered-carbon powder was applied as the negative electrode material in Li ion batteries. The thermal characteristics of the Si/C electrode mixed with or without the electrolyte were investigated by DSC. Both lithiated (with capacity of 1120mAhg−1) and delithiated Si/C electrode powder showed exothermic peaks in DSC curves due to the reactions between the Li in the electrode and the SEI. The lithiated electrode caused much larger exothermic heat than the delithiated one, especially when they were mixed with the electrolyte. By changing the ratio between the lithiated Si/C electrode and the coexisting electrolyte, the thermal behavior of the mixture could be studied in detail. At about 290°C, a mixture containing 0.5mg lithiated Si/C electrode and 0.5μl electrolyte showed the most drastic exothermic heat of 2.2J among all mixtures due to a direct reaction between the residual lithiated electrode and the solvent of the electrolyte after the thermal breakdown of the SEI. Under this condition, the heat value based on the electrode weight was 4.4Jmg−1, and that based on capacity was 5.1JmAh−1. These values of graphite were estimated to be 1.9Jmg−1 and 5.8JmAh−1, respectively. The generated heat was found to be very sensitive to the electrode/electrolyte ratio.