The reaction of ClO2 center dot with S2O32- in aqueous solution is a component of the "crazy clock" reaction of ClO2- with S2O32-, and under conditions of excess S2O32- the absorbance at 360 nm due to ClO2 center dot decays with sigmoidal kinetics. A chain reaction mechanism is inferred on the basis that very small concentrations of SO32- accelerate the reaction, and methionine inhibits the reaction. Pseudo-first-order kinetics is observed in the presence of relatively large methionine concentrations, leading to the simple rate law -d[CIO2]/dt = (k(a)[S2O32-] + k(b)[S2O32-](2))[ClO2], with k(a) = 452 +/- 16 M-1 s(-1) and k(b) = (5.7 +/- 0.2) X 10(5) M-2 s(-1) at 25 degrees C and pH 7.6. Under these conditions, the initial products are ClO2- and S4O62-. A classical electron-transfer mechanism is assigned to the reaction that occurs under conditions of methionine inhibition.