The rate of ClO2 conversion to ClO2- and ClO3- is accelerated by BrO2-, repressed by ClO2-, and greatly assisted by many nucleophiles (Br- > PO43- > HPO42- > CO32- > Cl- similar to OH-> CH3COO- similar to SO42- similar to C5H5N much greater than H2O). The kinetics (at p[H+] = 9.3-12.9) show that the first step of the mechanism is an electron transfer between ClO2 and BrO2- (k(1) = 36 M-1 s(-1)) to give ClO2- and BrO2. This highly reversible reaction (k(1)/k(-1) = 1 x 10(-6)) accounts for the observed inhibition by ClO2-, The second step is an electron transfer between ClO2 and BrO2 to regenerate BrO2- and form ClO3-. A novel aspect of the second step is the large kinetic contribution from nucleophiles (k(Nu)) that assist the electron transfer between ClO2 and BrO2. The k(Nu) (M-2 s(-1)) values at 25.0 degreesC vary from 2.89 x 10(8) for Br- to 2.0 x 10(4) for H2O.