Calcium-ion batteries (CIBs) have been attracting attention as next-generation secondary batteries. On the other hand, there are many problems to be studied such as the choice of electrolytes and electrode materials. In this study, we investigate the influence of the presence of water in electrolyte on the Ca2+ insertion/extraction properties of alpha-V2O5. Electrochemical tests showed that the overvoltage gradually decreases as the amount of water increases. Structural and elemental analyses suggested co-insertion of H+ in the water containing electrolyte and although the co-insertion amount of H+ differs depending on the amount of water, there is not much change in overvoltage reduction. So, the influence of H+ co-insertion on electrochemical properties is considered to be small. Bulk structure analysis of the electrolyte suggested that hydration to Ca2+ and dissociation of the contact ion pair of Ca2+ and anion progressed with increasing the amount of water in the electrolyte. From the above, it was revealed that the presence of water in electrolyte caused a large change in its structure, which has an important effect such as a reduction of overvoltage on electrochemical properties. (c) 2018 Elsevier Ltd. All rights reserved.