Reported here is the first example of a ruthenium complex, [Ru(III)(edta)(H(2)O)](-) (edta(4-) = ethylenediaminetetraacetate), that catalyzes the oxidation of thiourea (TU) in the presence of H(2)O(2). 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 (TU(2)) as a major product at the end of the catalytic process, however, formation of other products like thiourea dioxide (TUO(2)), thiourea dioxide (TUO(3)) 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 H(2)O(2) under the specified conditions.