The reaction of chlorine dioxide with excess NO2- to form ClO2- and NO3- in the presence of a large concentration Of ClO2- is followed via stopped-flow spectroscopy. Concentrations are set to establish a preequilibrium among ClO2, NO2-, ClO2, and an intermediate, NO2. Studies are conducted at pH 12.0 to avoid complications due to the ClO2-/NO2- reaction. These conditions enable the kinetic study of the ClO2 reaction with nitrogen dioxide as well as the NO2 disproportionation reaction. The rate of the NO2/ClO2 electron-transfer reaction is accelerated by different nucleophiles (NO2- > Br- > OH- > CO32- > PO4 (3-) > ClO2- > H2O). The third-order rate constants for the nucleophile-assisted reactions between NO2 and C102 (k(Nu), M (-2) s(-1)) at 25.0 degreesC vary from 4.4 x 10(6) for NO2- to 2.0 x 10(3) when H2O is the nucleophile. The nucleophile is found to associate with NO2 and not with ClO2 in the rate-determining step to give NuNO(2)(+) + ClO2. The concurrent NO2 disproportionation reaction exhibits no nucleophilic effect and has a rate constant of 4.8 x 10(7) M-1 s(-1). The ClO2/NO2/nucleophile reaction is another example of a system that exhibits general nucleophilic acceleration of electron transfer. This system also represents an alternative way to study the rate of NO2 disproportionation.