LiNi1/2Mn1/2O2 electrodes with layered structure were synthesized by solid-state reaction between lithium hydroxide and mixed Ni,Mn oxides obtained from co-precipitated Ni,Mn carbonates and hydroxides and freeze-dried Ni,Mn citrates. The temperature of the solid-state reaction was varied between 800 and 950°C. This method of synthesis allows obtaining oxides characterized with well-crystallized nanometric primary particles bounded in micrometric aggregates. The extent of particle agglomeration is lower for oxides obtained from freeze-dried Ni,Mn citrates. The local Mn4+ surrounding in the transition metal layers was determined by X-band electron paramagnetic resonance (EPR) spectroscopy. It has been found that local cationic distribution is consistent with α,β-type cationic order with some extent of disordering that depends mainly on the precursors used. The electrochemical extraction and insertion of lithium was tested in lithium cells using Step Potential Electrochemical Spectroscopy. The electrochemical performance of LiNi1/2Mn1/2O2 oxides depends on the precursors used, the synthesis temperature and the potential range. The best electrochemical response was established for LiNi1/2Mn1/2O2 prepared from the carbonate precursor at 900°C. The changes in local environment of Mn4+ ions during electrochemical reaction in both limited and extended potential ranges were discussed on the basis of ex situ EPR experiments.