We report the results of an experimental study of the oxidation of cysteinesulfinic acid (CysSO(2)H) by [IrCl6](2-) in aqueous media at 25 degrees C in order to gain insight into the mechanisms of oxidation of alkylsulfinic acids by simple one-electron oxidants. When the reaction is performed with exclusion of O-2 between pH 3 and 5, it is complete in several seconds. The products are [IrCl6](3-) and CysSO(3)H. Kinetic data obtained by stopped-flow UV-vis methods with [CysSO(2)H] >> [Ir-IV](0) reveal the rate law to be -d[Ir-IV] = k[Ir-IV](2)[CysSO(2)H]/[Ir-III] with a negligible pH dependence. The value of k is (6.8 +/- 0.12) x 10(3) M-1 s(-1) at mu = 0.1 M (NaClO4). A mechanism is inferred in which the first step is a rapid and reversible electron-transfer equilibrium between Ir-IV and CysSO(2)(-) to form Ir-III and CysSO(2)(center dot). The second step is the rate-limiting inner-sphere oxidation of CysSO(2)(center dot) by Ir-IV. Production of CysSO(3)H is proposed to occur through hydrolysis of an Ir-III-bound sulfonyl chloride that is the immediate product of the inner-sphere second step.