This article reports on the kinetics and mechanism of oxidation of the biologically active molecule guanylthiourea (GTU) by acidic bromate. This was a follow-up of a previous study in which mild oxidizing agents acidic iodate and molecular iodine oxidized GTU to a ring-cyclized product: 3,5-diamino-1,2,4-thiadiazole. In contrast, acidic bromate and molecular bromine, as stronger oxidizing agents, were able to oxidize GTU all the way to a complete desulfurization to yield guanylurea. No N-bromination was observed on any of the amino groups on guanylurea. The stoichiometry of the reaction was deduced to be 4BrO(3)(-) + 3H(2)N(=NH)CNH(C=S)NH2 + 3H(2)O --> 4Br(-) + 3H(2)N(=NH)CNH(C=O)NH2 + 3SO(4)(2-) + 6H(+). In excess bromate conditions in which the ratio of oxidant to reductant R = [Bro(3)(-)](0)/[GTU](0) > 1.6, the stoichiometry of the reaction was 8BrO(3)(-) + 5H(2)N(=NH)CNH(C=S)NH2 + H2O --> 4Br(2)(aq) + 5H(2)N(=NH)CNH(C=O)NH2 + 5SO(4)(2-) + 2H(+). The direct reaction of aqueous bromine with GTU was extremely fast with an estimated lower limit bimolecular rate constant of 7.5 +/- 1.2 x 10(4) M-1 s(-1). This rapid reaction with bromine produced reaction dynamics which involved bromine formation after a short induction period which was determined by the time it took for the complete oxidation of GTU and its oxidation intermediates. The mechanism of the reaction involved the initial formation of the ring-cyclized 3,5-diamino-1,2,4-thiadiazole which was later successively oxidized through the sulfoxide and sulfone, followed by the opening of the ring to yield sulfate and guanylurea. All the observed global reaction dynamics were satisfactorily modeled by a simple mechanism involving 14 elementary reactions.