The rate of metal-thiolate oxygenation by hydrogen peroxide has been measured for a model complex of nickel-containing superoxide dismutase (NiSOD) as 0.47 ± 0.03 M⁻¹ s⁻¹ at 303 K. Under typical synthetic conditions the reaction proceeds within seconds yielding S-oxygenate products. At biologically relevant H₂O₂ concentrations, half-life times extend to several hours or days. The results suggest H₂O₂ oxidation of NiSOD is not kinetically competent on the SOD timescale accounting for the lack of S-oxygenation byproducts.