The thermal stability of a charged graphite electrode was studied quantitatively by differential scanning calorimetry (DSC). Charged electrode powder gave exothermic peaks at around 285°C, and their heat values were proportional to the amount of charged electrode powder sealed in hermetic pans. These results suggest that a solid electrolyte interphase (SEI) that is formed on graphite during charging would react exothermally with charged graphite at around 285°C. Mass spectrometry coupled with thermogravimetric analysis and differential thermal analysis (TG-DTA/MS) of the charged electrode powder indicated that the exothermic reactions at around 285°C should be accompanied by the generation of methane. When charged electrode powders coexisted with electrolyte solution in a hermetic pan, the heat values at around 285°C varied in an apparently complicated way depending on the ratio of charged electrode powder to an electrolyte. These phenomena are discussed quantitatively by considering the amount of lithium-ions in charged graphite powder and a coexisting electrolyte. These results suggest that the exothermic reactions at around 285°C can be attributed to the reductive decomposition of SEI by charged graphite.