The reaction between chlorite and hydroxymethanesulfinic acid, HOCH2SO2H, (HMSA) has been studied in the pH range 4-8. The reaction is very complex and has a variable stoichiometry that is dependent on ratio of the oxidant to the reductant. In excess HMSA, the stoichiometry is ClO2- + HOCH2SO2H --> SO42- + HCHO + Cl- + 2H(+) while in excess ClO2- the stoichiometry is 3ClO(2)(-) + 2HOCH(2)SO(2)H --> 2SO(4)(2-) + 2HCOOH + 4H(+) + 3Cl(-). The reaction is very fast and is characterized by a short induction period (approximate to 1 s) which is followed by a rapid and autocatalytic ClO2 production. The first stage in the oxidation is the cleavage of C-S bond to form SO42- and HCHO. In the presence of excess oxidant the HCHO is oxidized to HCOOH. After prolonged standing (72 h or more), the HCOOH can be oxidized to CO2; giving a third possible stoichiometry. The rate-determining step is a 2-electron oxidation of HMSA by ClO2- to give the sulfonic acid HOCH2SO3H. A 19-reaction mechanism is used to simulate the reaction. There is reasonable agreement between the experiments and simulations.