It is shown that the Li+ ion in LiMn2O4 can be replaced by the Mg2+ ion, resulting in the formation of MgMn2O4 in aqueous Mg(NO3)(2) electrolytes by subjecting LiMn2O4 electrodes to cyclic voltammetry. The additional electronic charge due to the Mg2+ ion is compensated by a decrease in the average oxidation state of Mn from 3.5 in LiMn2O4 to 3.0 in MgMn2O4. Chemical analysis, powder X-ray diffraction, energy dispersive X-ray analysis and X-ray photoelectron spectroscopy studies confirm the presence of Mg in the compound. Cyclic voltammetry and chronopotentiometry studies suggest that MgMn2O4 can undergo reversible redox reaction by intercalation/deintercalation of Mg2+ ions in aqueous Mg(NO3)(2) electrolytes. (C) 2008 The Electrochemical Society.