The vanadium(IV) ion is found to form the [VO(SO4)(H2O)(4)]center dot H2O complex, as well as the dimer, [VO(H2O)(3)](2)(mu-SO4)(2), in concentrated H2SO4 media. Their formation mechanisms were investigated by UV-Visible spectroscopy (UV-Vis), Raman spectroscopy, X-ray diffraction (XRD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). UV-Vis spectroscopy study showed that [VO(SO4)(H2O)(4)]center dot H2O concentration in H2SO4 solution was proportional to concentrations of VO2+ and SO42-. The increased deviation from the near centrosymmetry of the octahedral complexes is due to the replacement of an equatorial water oxygen in [VO(H2O)(5)]SO4 by a sulfate oxygen in [VO(SO4)(H2O)(4)]center dot H2O. The dimer shows symmetrical structure, which correlates very well with non-activity in UV-Vis spectroscopic analysis. Structural information on both vanadium(IV) species can be confirmed by Raman and XRD measurements of crystals from the supersaturated solution of VOSO4 in 1 M, 6 M and 12 M sulfuric acid. A solution of vanadium(IV) (0.05 M) in 12M H2SO4, in which the vanadium(IV) species is [VO(H2O)(3)](2)(mu-SO4)(2), exhibits a reversible redox behavior near 1.14V (vs. SCE) on the carbon paper electrode. (C) 2011 Elsevier Ltd. All rights reserved.