Oxidation of thioglycolic acid (HSCH₂CO₂H, TGA) by [IrCl₆]²⁻ (Ir(IV)) in aqueous solution at 25 °C is highly susceptible to trace metal-ion catalysis. This catalysis can be effectively suppressed by bathophenanthrolinedisulfonate (bathophen), allowing the direct oxidation to be studied. Although the corresponding disulfide is produced quantitatively in the absence of bathophen, a mixture of the disulfide and a species believed to be the sulfonate is produced in its presence. In both cases the Ir(IV) is reduced to [IrCl₆]³⁻. The rate law when catalysis is suppressed is first order in both Ir(IV) and TGA. The pH dependence is complex, owing to the diprotic nature of TGA. At high pH there is a term corresponding to outer-sphere oxidation of ⁻SCH₂CO₂⁻ with a rate constant of (3.52 ± 0.03) × 10⁶ M⁻¹ s⁻¹ at μ  = 0.1 M. A self-exchange rate constant of 1 × 10⁵ M⁻¹ s⁻¹ for the ˙SCH₂CO₂⁻/⁻SCH₂CO₂⁻ redox couple is derived from the Marcus cross relationship. In order to account for the product mixture, a mechanism is proposed in which RSO₃⁻ is produced in a series of steps originating with the oxidation of RS˙ by [IrCl₆]²⁻, while RSSR is produced by association of RS˙ with TGA to generate RSSR˙⁻ and oxidation of RSSR˙⁻ by [IrCl₆]²⁻.