The thermal stability of electrolytes with Li x CoO2 cathode or lithiated carbon anode was reviewed including our recent results. From our experiments, it was found that Li x CoO2, delithiated by a chemical method using H2SO4 showed two exothermic peaks, one beginning at 190°C and the other beginning at 290°C. From high-temperature XRD, it was found that the first peak, from 190°C, was the phase transition from a monoclinic (R-3m) to a spinel structure (Fd3m). The spinel structure Li x CoO2 showed a very small cycling capacity. Probably, cation mixing was induced by the heat treatment. The DSC measurements of Li0.49CoO2 with 1M LiPF6/EC+DMC showed two exothermic peaks. The peak starting at 190°C probably resulted from the decomposition of solvent due to an active cathode surface, and the peak starting at 230°C was electrolyte oxidation caused by released oxygen from Li0.49CoO2. From DSC profiles of chemically delithiated Li0.49CoO2 and 1M PC electrolytes with various Li salts, it was found that the inhibition effect of the surface reaction starting at 190°C was large when LiBF4, LiPF6, and LiClO4 were used. The thermal stability of electrochemically lithiated graphite with 1M LiPF6/EC+DMC and PVdF-binder has been investigated. DSC revealed a mild heat generation starting from 130°C with a small peak at 140°C. The mild heat generation continued until a sharp exothermic peak appeared at 280°C. The lithiated graphite with the electrolyte without PVdF-binder did not show the small peak at 140°C. The peak at 140°C seems to be caused by the reaction (the solid electrolyte interphase (SEI) formation) of the electrolyte and lithiated graphite, which surface is covered by poly(vinylidene fluoride) (PVdF)-binder without formation of SEI at a lower temperature.