The oxidation of hydroxymethanesulfinic acid, HMSA, by acidic iodate has been studied by spectrophotometric techniques. The reaction presents clock reaction characteristics in which in excess iodate conditions there is an initial quiescent period that is followed by a rapid production of iodine. The induction period before formation of iodine is inversely proportional to the iodate- concentrations and the concentrations of acid to the second order. Iodide ions have a strong catalytic effect on the rate of the reaction by reducing the duration of the induction period. The stoichiometry of the reaction is dependent on the ratio of oxidant to reductant. In excess HMSA conditions the stoichiometry was deduced to be 3HOCH(2)SO(2)H + 2IO(3)(-) --> 3HCHO + 3SO(4)(2-) + 2I(-) + 6H(+) whereas in excess iodate and after prolonged standing the stoichiometry is 6IO(3)(-) + 5HOCH(2)SO(2)H --> 5SO(4)(2-) + 5HCOOH + 3I(2) + 4H(+) + 3H(2)O. The mechanism is dominated by the standard oxyiodine kinetics that involve the initial formation of the reactive oxyiodine species HIO2 and HOI: IO3- + 2H(+) + I- reversible arrow HIO2 + HOI. Further reactions will then occur between the organosulfur species with HOI. The direct reaction of aqueous iodine with HMSA is fast enough to be considered diffusion controlled, with a stoichiometry of 2I(2) + HOCH2SO2H + 2H(2)O --> HCHO + SO42- + 4I(-) + 6H(+). The facile nature of this reaction implies that HMSA and iodine cannot coexist in the reaction medium and that the end of the induction period coincides with a complete consumption of HMSA by iodate. The first 2-electron oxidation of HMSA yields a stable bisulfite addition compound, hydroxymethanesulfonic acid, HMSOA. Further oxidation of HMSOA is very slow, with the pathway involving the initial dissociation of HMSOA to HSO3- and HCHO. Although HSO3- is rapidly oxidized to SO42-, HCHO is only slowly oxidized to HCOOH.