A series of Li2MO3-x (M = Mn3/8Co1/4Ni3/8; 0 <= x <= 0.75) in the Li2MO3-Li2MO2 system was synthesized under high pressure, and their structures and electrochemical properties were investigated. X-ray diffraction measurements clarified the layered rock-salt-type structures in the composition range of x = 0-0.50. The cell parameters continuously increased with x, which indicates the change in the oxygen content of the products. During the first charge process, Li2Mn3/8Co1/4Ni3/8O3 showed an irreversible plateau region at 4.4 V, corresponding to electrochemical activation process. In contrast, the plateau region capacities drastically decreased with decreasing oxygen content. The oxygen-deficient phases are intrinsically electrochemically active without undergoing the activation process. The first discharge capacities increased with decreasing oxygen content from Li2Mn3/8Co1/4Ni3/8O3 (100 mAh g(-1)) to Li2Mn3/8Co1/4Ni3/8O2.57 (200 mAh g(-1)). The oxygen deficiency significantly affects the initial electrochemical reactions and the charge/discharge capacities at the subsequent cycles for the lithium-rich layered cathodes. (C) 2013 Published by Elsevier B.V.