A simultaneous electrochemical and calorimetric measurement was conducted on a spinel lithium manganese oxide (LiMn2O4)-cathode, carbon-anode prismatic lithium ion cell. From the comparison of thermal behavior at room temperature (25 degrees C) and at an elevated temperature (40 degrees C), a pronounced exothermic thermal behavior was observed during charging in the high voltage region (0.2 < x < 0.5 in LixMn2O4) at elevated temperatures. The degraded cell that was operated at 40 degrees C indicated that the capacity fading occurred mainly in the same high voltage region in advance of lower region. The irreversible thermal behavior observed by calorimetry clearly indicates the degradation of LiMn2O4 at elevated temperatures.