The kinetics and mechanism of the [Ru^III(edta)(H₂O)]⁻-mediated oxidation of cysteine (RSH) by hydrogen peroxide (edta⁴⁻ = ethylenediaminetetraacetate), were studied in detail as a function of both the hydrogen peroxide and cysteine concentrations at pH 5.1 and room temperature. The kinetic traces reveal clear evidence for a catalytic process in which hydrogen peroxide reacts directly with cysteine coordinated to the Ru^III(edta) complex in the form of [Ru^III(edta)SR]²⁻. A parallel process in which [Ru^III(edta)(H₂O)]⁻ first reacts with H₂O₂ to produce [Ru^V(edta)O]⁻ and subsequently oxidizes cysteine, is orders of magnitude slower than the [Ru^III(edta)(H₂O)]⁻-mediated oxidation in which cysteine rapidly coordinates to [Ru^III(edta)(H₂O)]⁻ prior to the reaction with H₂O₂. HPLC product analyses revealed the formation of cystine (RSSR) as major product along with cysteine sulfinic acid (RSO₂H) in the reaction system, and established the catalytic role of [Ru^III(edta)(H₂O)]⁻. Simulations were performed to account for the rather complex kinetic traces in terms of the suggested reaction mechanism. The results of the simulations support the proposed reaction mechanism that involves the oxidation of coordinated cysteine to cysteine sulfenic acid (RSOH), which subsequently rapidly reacts with H₂O₂ and RSH to form RSO₂H and RSSR, respectively.