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 O2 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.