Reported here is the first example of a ruthenium complex, [Ru^III(edta)(H₂O)]⁻ (edta⁴⁻ = ethylenediaminetetraacetate), that catalyzes the oxidation of thiourea (TU) in the presence of H₂O₂. The kinetics and mechanism of this reaction were investigated in detail by using rapid-scan spectrophotometry as a function of both the hydrogen peroxide and thiourea concentrations at pH 4.9 and 25 °C. Spectral analyses and kinetic data clearly support a catalytic process in which hydrogen peroxide reacts directly with thiourea coordinated to the Ru^III(edta) complex. HPLC product analyses revealed the formation of formamidine disulfide (TU₂) as a major product at the end of the catalytic process, however, formation of other products like thiourea dioxide (TUO₂), thiourea dioxide (TUO₃) and sulfate was also observed after longer reaction times. Catalytic intermediates such as [Ru^III(edta)(OOH)]²⁻ and [Ru^V(edta)(O)]⁻ were evidently found to be non-reactive in catalyzing the oxidation of thiourea by H₂O₂ under the specified conditions.