Thermal behavior during charge and discharge of mesocarbon microbeads (MCMB) hear-treated at 700, 1000, and 1800 degreesC was investigated by calorimetry to clarify the origin of the hysteresis in the voltage profiles of the former two kinds of MCMB. MCMB heat-treated at 700 and 1000 degreesC exhibited large exotherms over voltage plateaus at cell voltages close to 0 V during lithium insertion and at about 1 V during lithium extraction, which cannot be explained by polarization or entropy change. The large generation of heat indicated that some exothermic chemical reactions are associated with insertion/extraction reactions in the plateau regions in both directions. The anomalous thermal behaviors over the plateau regions were successfully explained by an energy diagram proposed by Zheng et al. [J. Electrochem. Soc., 143, 2137 (1996)] in which lithium insertion is followed by a chemical reaction that has an activation energy barrier.