The addition of chlorite to horse-radish peroxidase appears to initiate the same sequence of reactions as peroxide: the formation of a complex I followed by its transition to complex II. However, the amount of free chlorite or hypochlorite required to give half-maximal formation of the peroxidase complexes is over 100 times the amount of free hydrogen peroxide that is required. Complex I is not observed at an appreciable concentration because chlorite also acts as an electron donor (from which chlorine dioxide is formed) and accelerates the transition from complex I to II. The complex II formed from chlorite has the same reactivity toward nitrite as does that formed from peroxide, and the complexes may be considered to be identical. With hypochlorite, the formation of a complex I is readily observed, as is its transition to complex II. An accurate evaluation of the reactivity of this complex II toward donors has not yet been obtained.