The disproportionation of chlorine dioxide in basic solution to give ClO2- and ClO3- is catalyzed by OBr- and OCl-. The reactions have a first-order dependence in both [ClO2] and [OX-] (X = Br, Cl) when the ClO2-concentrations are low. However, the reactions become second-order in [ClO2] with the addition of excess ClO2, and the observed rates become inversely proportional to [ClO2-]. In the proposed mechanisms, electron transfer OX- ClO2 (k(1)(OBr-) = 2.05 +/- 0.03 M-1 s(-1) for OBr-/ClO2 and k(1)(OCl-) = 0.91 +/- 0.04 M-1 s(-1) for OCl-/ClO2) from OX- to ClO2 (k(1)(OBr-) = 2.05 +/- 0.03 M-1 s(-1) for OBr-/ClO2, and k(1)(OCl-) = 0.91 +/- 0.04 M-1 s(-1) for OCl-/ClO2) occurs in the first step to give OX and ClO2-. This reversible step (k(1)(OBr-)/k(-1)(OBr) = 1.3 x 10(-7) for OBr-/ClO2, k(1)(OCl-)/k(-1)(OCl) = 5.1 x 10(-10) for OCl-/ClO2) accounts for the observed suppression by ClO2. The second step is the OBr = 1.0 X 10(8) M-1 reaction between two free radicals (XO and ClO2) to form XOClO2. These rate constants are k(2)OBr = 1.0 x 10(8) M-1 s(-1) for OBr/ClO2. and k(2)(OCl) = 7 x 10(9) M-1 s(-1) for OCl/ClO2. The XOClO2 adduct hydrolyzes rapidly in the basic solution to give ClO3- and to regenerate OX-. The activation parameters for the first step are DeltaH(1)(double dagger) = 55 +/- 1 kJ mol(-1), DeltaS(1)(double dagger) = -49 +/- 2 J mol(-1) K-1 for the OBr-/ClO2 reaction and DeltaH(1)(double dagger) = 61 +/- 3 kJ mol(-1), DeltaS(1)(double dagger) = -43 +/- 2 J mol(-1) K-1 for the OCl-/ClO2 reaction.