The kinetics and mechanism of the reaction between Cl-2 and ClO2- are studied in acetate buffer by stopped-flow spectrometric observation Of ClO2 formation. The reaction is first-order in [Cl-2] and [ClO2-], with a rate constant of k(1) = (5.7 +/- 0.2) x 10(5) M-1 s(-1) at 25.0 degreesC. Nucleophilic attack by ClO2- on Cl-2, with Cl+ transfer to form CIOCIO and Cl-, is proposed as the rate-determining step. A possible two-step electron-transfer mechanism,for Cl-2 and ClO2- is refuted by the lack Of ClO2 suppression. The yield of ClO2 is much less than 100%, due to the rapid reactions of the metastable ClOClO intermediate via two competing pathways. In one path, CIOCIO reacts with ClO2- to form 2ClO(2) and Cl-, while in the other path it hydrolyzes to give ClO3- and Cl-. The observed rate constant also is affected by acetate-assisted hydrolysis of Cl-2. The rate of Cl-2 loss is suppressed as the concentration of Cl(-)increases, due to the formation, of Cl-3(-). In excess ClO2 a much slower formation Of ClO2 is observed after the initial Cl-2 reaction, due to the presence of HOCl, which reacts with H+ and Cl- to re-form steady-state levels of Cl-2.