The origin of the overcharge in the 5 V region observed in lithium-substituted LiMxMn2-xO4+delta spinels (M = Cr, Ni, Cu; x approximate to 0.2) prepared at 500degreesC was analyzed by using accurate analytical spectroscopic techniques (mass spectroscopy, nuclear magnetic resonance) to examine the electrolyte behavior. The spectra revealed organic solvents to be stable as no decomposition products were detected, thus excluding the electrolyte oxidation as a side reaction accounting for the cell overcharge. However, these spinels contain excess oxygen in an amount that was quantified from thermogravimetric data. The excess oxygen plays a prominent role in the electrochemical response of the spinel. The cyclic voltammetry and galvanostatic results support the assumption that the excess oxygen can be released above 4.5 V. The additional capacity obtained and that required to release the oxygen were quite consistent. This must be the origin of both the overcharge and the poor performance of the cells compared with spinels of similar composition but synthesized at higher temperatures (800degreesC), the excess of oxygen in which was smaller. (C) 2004 The Electrochemical Society.