Structural changes of LiMn2O4 spinel electrodes have been investigated in 4 V Li/LixMn2O4 cells by X-ray diffraction during prolonged electrochemical cycling between 4.5 and 3.4 V at room temperature. The capacity fade that is observed during cycling is attributed, at least partly, to disproportionation reactions at the particle surface of discharged LixMn2O4 electrodes, resulting in a soluble MnO component. The disproportionation reactions are described with respect to the Li-Mn-O phase diagram. The data support recent reports that Li2Mn2O4 can be formed at the electrode surface slightly above 3 V when cells are discharged and charged at high rates.