The reaction of chlorite and formaldehyde was studied in basic and slightly acidic media. Though the expected product was CO2, the oxidation of HCHO, however, gave nearly quantitative formation of ClO2, the oxidation product of ClO2-. In excess HCHO the stoichiometry of the reaction was deduced as 3ClO(2)(-) + HCHO + 2H(+) --> HCOOH + 2ClO(2)(aq) + Cl- + 2H(2)O; but in high excess of ClO2- the stoichiometry was 6ClO(2)(-) + HCHO + 4H(+) --> CO2(g) + 4ClO(2)(aq) + 3H(2)O + 2Cl(-). The reaction is autocatalytic in HOCl. The first step of the reaction produces HOCl, which catalyzes the formation of ClO2 and further oxidation of HCOOH to CO2. ClO2 was found to be relatively unreactive toward HCHO and HCOOH, and hence it accumulated rapidly.