We investigated batteries using calcium as the transfer ion with a view to developing high energy density and safe ion-transfer type batteries as an alternative to lithium-ion batteries. The electrochemical behavior of calcium electrodes in various kinds of electrolyte solution was investigated by means of galvanostatic measurements. The anodic dissolution proceeded easily, but there was no cathodic deposition in the solutions we tested in this study. As a result, we used Ca(ClO4)(2) solute/acetonitrile solution to evaluate the calcium insertion behavior. Moreover, we examined electrochemical insertion and extraction reactions with respect to calcium ions using vanadium oxides as calcium host materials. Their discharge curves indicated that a calcium insertion reaction did indeed occur. Structural changes in vanadium oxides resulting from the insertion and extraction of calcium ions were observed for the first time. (C) 2003 Elsevier Science B.V. All rights reserved.