<p>[Ru<sup>III</sup>(EDTA)(H<sub>2</sub>O)]<sup>−</sup> (EDTA<sup>4−</sup> = ethylenediaminetetraacetate) catalyzes the oxidation of biological thiols, RSH (RSH = cysteine, glutathione, N-acetylcysteine, penicillamine) using H<sub>2</sub>O<sub>2</sub> as precursor oxidant. The kinetics of the oxidation process were studied spectrophotometrically as a function of [Ru<sup>III</sup>(EDTA)(H<sub>2</sub>O)]<sup>−</sup>, [H<sub>2</sub>O<sub>2</sub>], [RSH], and pH (4–8). Spectral analyses and kinetic data are suggestive of a catalytic pathway in which the RSH reacts with [Ru<sup>III</sup>(EDTA)] catalyst complex to form [Ru<sup>III</sup>((EDTA)(SR)]<sup>2−</sup> intermediate species. In the subsequent reaction step the oxidant, H<sub>2</sub>O<sub>2</sub>, reacts directly with the coordinated S of the [Ru<sup>III</sup>((EDTA)(SR)]<sup>2−</sup> intermediate leading to formation of the disulfido (RSSR) oxidation product (identified by HPLC and ESI-MS studies) of thiols (RSH). Based on the experimental results, a working mechanism involving oxo-transfer from H<sub>2</sub>O<sub>2</sub> to the coordinated thiols is proposed for the catalytic oxidation.</p
