The mechanism of oxidation of ArSO2- (PhSO2- and 5-sulfinato-2-nitrobenzoic acid = TNBO21-/2-) with HOCl/OCl- has been investigated using the kinetic method. In contrast to previous reports for PhSO2- (for which it was suggested that OCl- and not HOCl was the reactant), the reaction proceeds through it conventional pathway: nucleophilic attack by ArSO2- on HOCl with concomitant Cl+ transfer to give a sulfonyl chloride intermediate (ArSO2Cl), which we have identified spectrophotometrically. Remarkably, the rate constant for the reaction of HOCl with ArSO2- is on the order of 10(9) M-1 s(-1), larger than the rate constants for corresponding thiolates, and is nearly diffusion-controlled. In contrast, the rate constant for the reaction of OCl with ArSO2- is approximately 7 orders of magnitude smaller.