Thermal reactions of charged Li x CoO2 and electrolyte are investigated by means of temperature-programmed decomposition–mass spectrometry (TPD–MS), DSC, TGA, and XRD. The electrolyte is composed of ethylene carbonate, propylene carbonate, and dimethyl carbonate. Direct observation of gas species resulting from the reactions is beneficial in understanding the reaction mechanisms. From O2 peaks in the TPD–MS spectra with weight loss in TGA and XRD results, it is obvious that the reduction of Li x CoO2 to LiCoO2 and Co3O4 is triggered at 190°C and completed at 400°C. The initial stages of electrolyte combustion and decomposition are confirmed by H2O and CO2 peaks in the TPD–MS spectra along with exothermic peaks in DSC plots. The reaction of electrolyte with O2 released from the reduction of Li x CoO2 begins at 240°C; the decomposition starts at 290°C.