We investigated the relation between the thermodynamic stability and cathode performance of lithium manganese spinels as cathode active materials for the lithium secondary batteries. The heat of dissolution of these samples was measured by the solution calorimetry method. The standard enthalpy of formation, Δ_fH⁰, the enthalpy change of reaction, ΔH, and the enthalpy change per mole of atoms for the formation reaction, ΔH_R, were calculated from the heat of dissolution. The larger │ΔH│ or │ΔH_R│ means a thermodynamically more stable sample. A more thermodynamically stable sample makes a stable crystal structure and leads to a good cycle performance.