The thermal stabilities of 1M LiPF6/EC+DEC and with electrodes were studied by calorimetry. The results show that both the electrolyte–Li0.5CoO2 and electrolyte–Li x C6 system have lower decomposition onset temperatures than either the separate electrolyte or electrodes. The electrolyte is oxidized by Li0.5CoO2, while its reaction with lithiated graphite occurs because the solid electrolyte interphase (SEI) breaks down at 57°C. The 1M LiPF6/EC+DEC in air is less stable than in argon, but the reaction is similar.