The kinetics of bromide oxidation by (oxalato)oxodiperoxomolybdate(VI), MoO(O-2)(2)(C2O4)(2-) were studied. At 25 degrees C and pH 5.0, the reaction obeys the rate law d[Br+]/dt = k(Br),[MoO(O-2)(2)(C2O4)(2-)][Br-], where k(Br), = (4.8 +/- 0.4) x 10(-3) M(-1) s(-1), measured by the production of bromophenol blue from phenol red. The latter reaction was previously used for bromine analysis and as an assay for bromoperoxidase enzyme activity; we now employ it to quantify the initial rate of bromine production at pH 5.0. The oxidation of bromide by MoO(O-2)(2)(C2O4)(2-) is catalytic in the presence of excess hydrogen peroxide and bromide. We propose a mechanism that involves the interaction of MoO(O-2)(2)(C2O4)(2-) and bromide to yield Br+ and a monoperoxo intermediate, MoO2(O-2)(C2O4)(2-); the monoperoxo complex combines with hydrogen peroxide to regenerate the initial diperoxo complex or undergoes hydrolysis to yield the final molybdenum product, MoO3(C2O4)(2-) The primary bromine product is unknown; "Br+" represents an equilibrium mixture of hypobromous acid, hypobromite, bromine, and tribromide. In the absence of an organic substrate, dioxygen is produced from the oxidation of hydrogen peroxide by Br+. The aqueous chemistry of the vanadium(V) and tungsten(VI) analogues is not well defined. Neither VO(O-2)(2)(C2O4)(3-) nor WO(O-2)(2)(C2O4)(2-) was Obtained as the sole species in solution at pH 5.0. The monoperoxo bis(oxalato) complex VO(O-2)(C2O4)(2)(3-) reacted with bromide at a rate indistinguishable from that of the uncatalyzed case at pH 5.0 and 25 degrees C. The diperoxo tungsten complex WO(O-2)(2)(C2O4)(2-) reacted with bromide at a rate faster than that for its molybdenum(VI) counterpart, but a complete kinetic analysis-was precluded by the presence of the unchelated complex WO(OH)(O-2)(2)(H2O)(-).