MG(2+) intercalation property of V2O5/carbon composites and accompanied structural changes in V2O5 of the composites were evaluated by cyclic voltammetry, X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR). Voltammogram of Mg2+ insertion process was similar to that of Li+, which has two main broad peaks. Ex-situ FT-IR spectra suggested that Mg2+ is inserted into innerlayer site of V2O5 at the first peak around -0.15 V vs. Ag/Ag+, and interlayer site at the second peak around -0.65 V vs. Ag/Ag+. Reversible structural changes of V2O5 xerogel accompanied with Mg2+ intercalation/deintercalatioin were confirmed by XRD. In comparison with conventional mixture electrode of V2O5 xerogel and carbon, the composite showed high capacity in the second peak. It is assumed that the Mg2+ diffusion in the interlayer site is slower than that of innerlayer site in the V2O5 xerogel. However, the interlayer site could be utilized effectively in the V2O5/carbon composite since the composite has large interlayer distance and short diffusion length compared to normal xerogel of V2O5. (C) 2003 Elsevier B.V. All rights reserved.