The reversible redox reaction of oxygen ion was directly observed by soft X-ray absorption spectroscopy for Li1.16Ni0.15Co0.19Mn0.50O2, the Li-rich manganese layered oxide. The O K-edge spectra indicated that a peroxide phase (namely the formation of O-2(2-)) appeared and disappeared reversibly by charging and discharging. It was simultaneously observed both in the total electron yield mode and in the partial fluorescence X-ray yield mode, indicating that the peroxide phase exists stably in the crystalline network. These species should play an important role in the high capacity positive electrode. (C) 2014 Elsevier B.V. All rights reserved.