The thermal stability of the materials that comprise the battery has been one of the important issues. By using temperature programmed desorption-mass spectrometry (TPD-MS) and XRD, the thermal decomposition reaction of delithiated Li x CoO2 (x =1, 0.81, 0.65) was quantitatively analyzed. Delithiated Li x CoO2 samples were metastable and liberated oxygen at a temperature of above 250°C. Liberated oxygen gas was quantified by TPD-MS. Structural changes of the samples were confirmed by XRD. We identified the stoichiometry of the thermal decomposition reaction of Li x CoO2. Furthermore, to analyze the heating rate dependence of the oxygen generation, we calculated the activation energy (E a) of the thermal decomposition reaction. The average E a through the reaction of Li0.81CoO2 is 130kJmol−1, and that of Li0.65CoO2 is 97kJmol−1. The Li content decreased as the activation energy increased.