The electrochemical insertion/extraction of calcium ions into/from crystalline vanadium oxide (c-V2O5) was investigated by galvanostatic measurements. Electrochemical cells using c-V2O5 as a cathode active material provided a discharge capacity of more than 450 mAh g(-1) at a current density of 50 muA cm(-2). However the discharge capacity decreased greatly as the current density increased. This behavior seemed to reflect that calcium ions do not diffuse easily in the oxide lattice. The structural changes in c-V2O5 after discharging and charging were analyzed by X-ray diffraction (XRD). After discharging, a new phase caused by the ion-insertion reaction was observed in the XRD pattern that coexisted with the pristine c-V2O5 phase. (C) 2004 The Electrochemical Society.