The thermal stabilities of 1 M LiPF6/EC+DEC and with electrodes were studied by calorimetry. The results show that both the electrolyte-Li0.5CoO2, and electrolyte-LixC6 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 degrees C. The 1 M LiPF6/EC + DEC in air is less stable than in argon, but the reaction is similar. (C) 2005 Elsevier B.V All rights reserved.