Li1+x[Ni1/3Co1/3Mn1/3](1-x)O-2/ Li half-cells were investigated using isothermal microcalorimetry (IMC), and the results were quantitatively analyzed. The entropy change (dE/dT) calculated based on the IMC results during the charging process has the small range from -0.06 mV to -0.12 mV/K, which agrees with the experimental results. The small dE/dT values could explain why Li1+x[Ni1/3Co1/3Mn1/3](1-x)O-2 has better thermal stability than that of other nickel based layered oxide such as LiNiO2, LiNi1-xCoxO2, and LiNi1-xCoyAlzO2. Furthermore, the heat flow rate during the discharge process was also calculated from the cell's irreversible heat and the cell entropy change. The calculated values fit well the experimental data from the IMC results. This study also suggests the optimum voltage window of the operation of the cell based on Li1+x[Ni1/3Co1/3Mn1/3](1-x)O-2 cathode is between 4.1 and 3.5 V due to both, the self-discharge of the cell at higher voltages and the higher impedance of the cell in lower voltage regions. (c) 2006 The Electrochemical Society.