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 β’ 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 β’ 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 β’ 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 β’ 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 β’ C can be attributed to the reductive decomposition of SEI by charged graphite.