The Mn2+-catalyzed oxidation of HSO3- by O-2 has been studied in the pH region 4.5 and at bisulfite ion concentrations from 1.5 x 10(-3) to 1.2 x 10(-2) M. The reaction was found to obey a three-term rate law : -d[O-2]/dt = k(alpha)[HSO3-](2) + k(beta)[HSO3-][Mn2+] + k(gamma)[Mn2+](2) with k(alpha) = 3.6 x 10(-3) M(-1) s(-1), k(beta) = 1.23 M(-1) s(-1), and k(gamma) = 98.6 M(-1) s(-1) at pH 4.50, 25 degrees C, and ionic strength 0.050 M. The kinetic behavior of the reaction resembles markedly that of the uncatalyzed reaction. The rate of the reaction is independent of oxygen concentration, S2O72- and HSO5- are intermediates in the reaction, and the reaction is catalyzed by S2O82- and strongly inhibited by methanol, The experimental results can be quantitatively explained by the addition to the uncatalyzed reaction mechanism of a chain propagation reaction involving Mn2+ and SO5.-. Among several alternatives, the manganese propagation may be represented as follows : (I) Mn2+ + SO5.- --> Mn(III) + HSO5-; (II) Mn(III) + HSO3- --> Mn2+ + SO3.-. The resulting mechanism leads to the three-term rate law where the first term is the uncatalyzed rate and the second term can be predicted quantitatively from the uncatalyzed rate and the last term. It was inferred from the yield of S2O72- that, unlike the reaction between SO5.- and HSO3-, there is little or no branching of the first reaction to form the SO4.- radical. The ratio of the rate constant of (I) to that of the reaction of SO5.- with HSO3- in the uncatalyzed reaction mechanism has been determined to be 124. The quantitative agreement between the experimental data and predictions for the effect of S2O82- has substantiated the validity of the proposed mechanism. The pH dependence of the reaction rate can be almost entirely accounted for by the pH dependence of the reaction between HSO5- and HSO3-, indicating that reaction I is independent of the hydrogen ion concentration in the pH region studied. While methanol inhibits the reaction, the quantitative discrepancy between predictions and experiments suggests that the reactions with alcohols are more complex than previously thought, Further work in this area is needed to understand fully the reaction mechanism when alcohol is present.