The electrochemical extraction/insertion of Li from/into LiCoMnO4 in the potential range of 3.7-5.25 V is studied by electron paramagnetic resonance (EPR) of Mn4+. Structural characterization of initial LiCoMnO4 has been performed by neutron diffraction. The EPR spectra are consistent with a single-phase mechanism of electrochemical extraction/insertion of Li from/into LiCoMnO4. During Li extraction, the intensity of the main EPR signal due to Mn4+ ions in Mn4+/Co3+ environment decreases drastically concomitantly with the appearance of resonance absorption from magnetically correlated Mn4+-Co4+ spins. From EPR, the complex [Con+-Mn3+-Mn4+] Clusters indicative of the oxygen non-stoichiometry of LiCoMnO4 also take part in the electrochemical reaction, especially up to 4.7 V. During the Li reinsertion, the EPR signal from Mn4+ ions in Mn4+/Co3+ environment is recovered, but its intensity remains lower (30%) as compared to that of the initial composition, which is consistent with an unrecovered composition of LiCoMnO4 electrode after first charge/discharge. Contrary to the narrow main signal, there is a reverse reduction of the paramagnetic ions present in the clusters, but their oxidation state as compared to the initial composition is not recovered. (c) 2005 Elsevier B.V. All rights reserved.