The stability of lithium manganese oxide spinel in LiPF6/EC:DMC (ethylene carbonate-dimethyl carbonate) based electrolytes has been investigated at different states of charge and at temperatures between 21 and 55 degrees C. Reversible capacity loss due to reinsertion of lithium takes place at all states of charge and temperature. This reaction can proceed to the extent of inserting more than one Li per Mn2O4 at 55 degrees C resulting in the formation of tetragonal Li1+xMn2O4. The second process, irreversible capacity loss due to dissolution of the cathode, makes little contribution to capacity loss in highly charged electrodes, but has an effect comparable to reinsertion in discharged electrodes.