Stokes radii of V(IV) and V(V) species in concentrated sulfuric acid solutions were determined from their diffusion limited current densities on a rotating platinum disk electrode and the solution viscosity. In addition, V(IV) and V(V) species were estimated based on their solubility, UV-Vis spectra, and cyclic voltammetric data. The possible ion-pair formation of V(IV) cation with SO42- and/or HSO4- and the spontaneous polymerization of V(V) at a low H2SO4 concentration were discussed. (C) 2004 Elsevier Ltd. All rights reserved.