Electrochemical behavior and the variation of electronic structure upon lithium insertion into the A-site-deficient perovskite oxide, Gd-1/3 TaO3, have been studied by X-ray absorption spectroscopy (XAS). During electrochemical lithium insertion, Gd L-III-edge and Ta L-1-edge XAS revealed that Gd ion did not contribute the charge compensation, while Ta ion reduced its oxidation state. Furthermore, the changes in O K-edge spectra with electrochemical lithium insertion were clearly observed, indicating that O ions also contribute to the electron-transfer reaction. (C) 2004 Elsevier B.V. All rights reserved.