It is well known that charged LixCoO2 (x<1) is metastable, and that oxygen evolution has been observed at temperatures above 200 degreesC. LixCoO2, delithiated by a chemical method using H2SO4, was investigated by means of differential scanning calorimetry (DSC) with/without an electrolyte (1 M LiPF6/ethylene carbonate (EQ + dimethyl carbonate (DMC). The lithium content x in the delithiated LixCoO2, was determined by atomic absorption spectroscopy. The DSC profile of Li0.49CoO2 showed two exothermic peaks, one beginning at 190 degreesC and the other beginning at 290 degreesC. From high-temperature X-ray diffraction (XRD), it was found that the first peak, from 190 degreesC, was the phase transition from a monoclinic (R (3) over barm) to a spinel structure (Fd3m). The DSC measurements of Li0.49CoO2 With the electrolyte at various mixing ratios showed two exothermic peaks, one beginning at 190 degreesC and the other at 230 degreesC. The exothermic heat of each peak was proportional to the amount of Li0.49CoO2. The peak starting at 190 degreesC probably resulted from the decomposition of solvent due to an active cathode surface, and the peak starting at 230 degreesC was electrolyte oxidation caused by released oxygen from Li0.49CoO2. The exothermic heat from 190 to 230 degreesC based on cathode weight was 420 +/- 120 J/g, and that from 230 to 300 degreesC was 1000 +/- 250 J/g.