Although Li-rich solid-solution layered materials Li2MnO3-LiMO2 (M = Co, Ni, etc.) are expected as large capacity lithium insertion cathodes, the fundamental charge-discharge reaction mechanism of these materials is not clear. Therefore the change in valence states of Ni, Co and Mn of Li[Ni0.17Li0.2Co0.07Mn0.56]O-2 during charge-discharge was examined in detail using in situ X-ray absorption spectroscopy (XAS), which includes both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Since the Mn K edge shift during charge-discharge was not clear to determine the valence change of Mn, the Mn K pre-edge shift was examined during charge-discharge. In our measurements, only a small shift of the Mn K pre-edge toward lower energy was observed on discharge from 4.8 to 2.0V. This corresponds to a decrease of the Mn valence from 4+ to approximately 3.6+. However, this shift cannot explain the large reversible capacity of this material and thus strongly suggests the participation of oxygen in the reversible charge-discharge reaction of this material. (C) 2010 Elsevier B.V. All rights reserved.