Electron paramagnetic resonance spectroscopy was used to analyse the changes in local Ni, Mn environment in layered LiNi1/2Mn1/2O2 and spinel LiNi1/2Mn3/2O4 after electrochemical extraction and reinsertion of lithium. For layered LiNi1/2Mn1/2O2, the EPR signal from Mn4+ is only detected, while residual antiferromagnetic correlations between Ni2+ and Mn4+ ions gives rise to strong resonance absorption for LiNi1/2Mn3/2O4 spinel. The first charge process of layered LiNi1/2Mn1/2O2 leads to oxidation of Ni2+ ions located both in the transition metal sites and in the Li sites. The reverse process of reduction of these nickel ions was suggested to proceed between 4.4–3.0V and 2.5–1.4V, respectively. Lithium extraction from LiNi1/2Mn3/2O4 spinel leads to oxidation of paramagnetic Ni2+ to diamagnetic Ni4+ without significant changes in the local environment of Mn4+. For both fully delithiated compositions, Li1−x Ni1/2Mn1/2O2 and Li1−x Ni1/2Mn3/2O4, an EPR spectrum from localized Mn4+ ions is observed, indicating an exhaustion of paramagnetic Ni2+ ions in the vicinity of Mn4+ ions. Furthermore, it has been found that the Mn4+ environment including paramagnetic Mn4+ and Ni2+ neighbours is restored after the first cycle of charge/discharge.