An electroactive spinel solid solution, LiFexMn2-xO4 (0 less than or equal to x less than or equal to 0.5). was prepared and investigated. X-ray Rietveld refinements showed a small amount of transition metal ions in tetrahedral 8a sites in Fe-substituted samples. The Fe-57 Mossbauer spectrum of LiFe0.5Mn1.5O4 at 300 K is composed of two doublets and attributed to trivalent Fc. In the voltage range 3.0 and 5.3 V vs. Li/Li+. two reversible plateaus appeared at 4.9 and 3.9 V during discharge. Combined data of X-ray diffraction, Mn K-edge X-ray absorption near-edge structure (XANES), and Fe-57 Mossbauer spectroscopy obtained in situ during charge and discharge demonstrate that the plateau around 4 V on charging is associated with the oxidation of Mn3+ to Mn4+ and the plateau around 5 V with the partial oxidation of Fe3+ to Fe4+.