Bromide oxidation by hydrogen peroxide is catalyzed by cis-dioxovanadium(V) in acidic, aqueous and aqueous/ethanolic solution. Hydrogen peroxide consumption is stoichiometric with bromide oxidation. Bromide is oxidized by two electrons and can undergo further reaction with organic substrates or hydrogen peroxide. The dependence of the rate of bromide oxidation is second order in vanadium concentration and first order in bromide concentration. Coordination of hydrogen peroxide by vanadium(V) gives rise to an equilibrium mixture of oxomonoperoxo- and oxodiperoxovanadium(V), whose composition is dependent on the concentrations of acid and hydrogen peroxide. Kinetic and spectroscopic analyses suggest that catalysis occurs by dimerization of oxomonoperoxo- and oxodiperoxovanadium(V) to form a binuclear, triperoxo vanadium(V) complex followed by oxidation of bromide. Ethanol increases the rate of the reaction by increasing the formation of the vanadium(V) dimer. The bimolecular rate of bromide oxidation by the vanadium dimer is 4.1 +/- 0.1 M(-1) s(-1). The relevance of this catalytic system to vanadium bromoperoxidase and to other metal-catalyzed halide oxidations is discussed.