Molybdenum(VI) and tungsten(VI) catalyze the oxidation of bromide by hydrogen peroxide and thus are functional mimics of vanadium bromoperoxidase. Relatively few functional mimics are known. The oxidation of bromide by the oxodiperoxo complexes of molybdenum(VI) and tungsten(VI) has been investigated as a function of hydrogen peroxide, acid, bromide, and molybdenum(VI) or tungsten(VI) concentration. The oxidized bromine species were trapped by bromination of trimethoxybenzene (TMB) or followed by the formation of tribromide. The rate of bromide oxidation by the oxodiperoxometalate(VI) species is first order in bromide and Mo(VI) or W(VI). The rate constant for bromide oxidation by MoO(O2)2(H2O)2 is 1.51 X 10(-2) M-1 s-1 and by MoO(O2)2(H2O)(OH)- is 2.39 x 10(-3) M-1 s-1; the K(a) of MoO(O2)2(H2O)2, determined spectrophotometrically, is 5.0 X 10(-3) under the conditions employed (i.e., 10(-4)-0.1 M HClO4 in 25% MeOH). Under conditions of excess hydrogen peroxide the oxidation of bromide is fully catalytic in molybdenum(VI) or tungsten(VI); coordination of H2O2 is fast compared to bromide oxidation. A catalytic cycle is proposed. Chloride was found to inhibit bromide oxidation by MoO(O2)2(H2O)2 but not by the oxalato complex, MoO(O2)2(ox)2- or WO(O2)2(H2O)2. The shift in lambda(max) from 324 to 329 nm upon chloride addition to MoO(O2)2(H2O)2 indicates that chloride coordinates to the MoO(O2)2 moiety. Spectral changes on chloride addition to MoO(O2)2(ox)2- and WO(O2)2(H2O)2 were not observed. The Mo(VI)- and W(VI)-catalyzed peroxidation bromination reactions are compared to the halogenation reactivity of VO2+ and vanadium(V) bromoperoxidase and to the oxygen atom transfer reactivity of oxoperoxomolybdenum(VI) and -tungsten(VI) complexes.