The oxidation of guanylthiourea, GTU, by mildly acidic iodate and molecular iodine has been studied. Its reaction with iodate shows an oligooscillatory formation and consumption of iodine in batch conditions. The major oxidation product is a ring-cyclized product of guanylthiourea, 3,5-diamino-1,2,4-thiadiazole (GTU-C), in which the thioureado moiety is oxidized to the unstable sulfenic acid that instantly attacks the distal amino group, eliminating water and forming the five-membered thiadiazole group. In excess GTU conditions, the stoichiometry of the reaction was 1:3 without any further oxidation past a 2-electron transfer, IO3- + 3H(2)NC(=NH)NH(C=S)NH2 --> I- + 3GTU-C + 3H(2)O, whereas in excess iodate conditions the stoichiometry is 2IO(3)(-) + 5H(2)NC(=NH)NH(C=S)NH2 + 2H(+) --> 5GTU-C + I-2(aq) + 6H(2)O. In high acid environments the reaction shows two peaks in iodine concentrations in batch conditions, and at lower acid concentrations one observes an induction period followed by a monotonic formation of iodine according to the Dushman reaction. The overall reaction is heavily catalyzed by iodide. The direct reaction of iodine and GTU is fast, with a bimolecular rate constant of (1.10 +/- 0.20) x 10(4) M-1 s(-1). This reaction is autoinhibitory with the product, iodide, inhibiting the reaction by forming the relatively inert I-3(-) species. Acid also inhibits the oxidation of GTU by iodine by protonating the thiol group, thereby reducing its nucleophilicity. A simple mechanistic scheme comprising 9 elementary and composite reactions was found to be adequate in explaining the full reaction scheme.