Spinel oxides with LiMxMn2-xO4 (M = Fe, Ni) formula are known to be interesting electrode materials for lithium-ion batteries. These materials show a significant near 5 V capacity, which results from changes in the oxidation state of the transition metal M. In this work, compounds of the LiNi0.5-yFeyMn1.5O4 series have been characterized. The charge/discharge curves of lithium test cells using these compounds as active electrode material show two plateaus. The first one, below 4.5 V, is ascribed to a Mn4+/Mn3+ pair, while the second, between 4.5 and 5.1 V, can be attributed to Ni4+/ Ni2+ and Fe4+/ Fe3+ pairs. High capacity and excellent capacity retention is obtained for some compositions with intermediate y values. The suppression of the multiphase mechanism of insertion-deinsertion commonly found for LiNi0.5\Mn1.5O4 by iron substitution is evidenced for compositions in the 0, y less than or equal to 1 range. The change in mechanism is correlated with the improvement in electrochemical performance. Moreover, cation distribution, which includes the presence of iron in the tetrahedral sites of the spinel structure, stabilizes the solid upon prolonged cycling. (C) 2004 The Electrochemical Society.