Spectroscopic changes in highly concentrated vanadium(V)-sulfate solutions to be used in the 'vanadium redox battery' are consistent with the presence of more than one V(V)-sulfate species. The results of Raman spectroscopy indicate that the major species in highly acidic conditions are VO2SO4-, VO2(SO4)(2)(3-), VO2(HSO4)(2)(-), VO3-, V(V) dimers with V2O34+ and V2O42+ central units. The nature and amount of these species depends upon the V(V) and total sulfate concentrations as well as on S to V and H+ to V ratios in the positive half-cell electrolyte. V(V) forms V2O34+, VO2(SO4)(2)(3)- and their copolymer species at higher total sulfate concentrations, which tends to stabilize the vanadium (V) positive electrolyte in the vanadium redox battery. The V(V) and V(IV) species show the least interaction with each other. Ageing of concentrated V(V) solutions at elevated temperature (50 degreesC) produces decomposition of species causing formation of V2O5 precipitates with a decrease in the amount of vanadium polymer.