The heat generation of mesocarbon microbeads (MCMB) in a MCMB/Li cell was measured in an isothermal microcalorimeter. The heat contributions are discussed in terms of reversible and irreversible heat generation. Endothermic and exothermic events observed during the charge and discharge processes are attributed to the stage changes of graphite during the electrochemical lithium insertion-extraction reactions. The initial endothermic peak is attributed to the insertion of the lithium ion into the graphite layer structure; the graphite structure changes from a dilute stage 1' to stage 4 with a positive DeltaS (due to the order/disorder transition). In addition, the heat generation is estimated on the basis of the cell entropy change and cell polarization at a low discharge rate. The open-circuit voltage and DeltaDS calculated from the isothermal calorimeter results under low charge/discharge rates have the same range and similar shapes when compared to the experimental results. (C) 2003 The Electrochemical Society.