Catalytic ability of [Ru-lll(edta)(H2O)](-) (edta(4-) = ethylenediaminetetraacetate) complex toward oxidation of thiosulfate (S2O32-) in presence of H2O2 has been explored in the present work. The kinetics of the catalytic oxidation of thiosulfate (S2O32-) has been studied spectrophotometrically as a function of [Rulll(edta)], [H2O2], [S2O32-] and pH. Spectral analyses and kinetic data indicate a catalytic pathway involving activation of both substrate (S2O32-) and oxidant (H2O2). Substrate activation pathway involves the formation of a red [Ru-lll(edta)(S2O3)](3-) species through the reaction of the [Ru-lll(edta)(H2O2)](-) catalyst complex and the substrate (S2O32-). Hydrogen peroxide reacts directly with thiosulfate coordinated to the Rum(edta) complex to yield sulfite as immediate oxidation product. Peroxide activation pathway is governed by the formation of [Ru-v(edta)(0)](-) catalytic intermediate which oxidize thiosulfate, however, at slower rate (k(ox)(2) = 0.012 M-1 s(-1) at 25 degrees C) as compared to the rate of oxidation of the coordinated thiosulfate (K-ox(1) = 0.93 M-1 s(-1) at 25 degrees C). Sulfite and sulfate were found to be the oxidation products of the above described catalytic oxidation process. A detailed mechanism in agreement with the spectral and kinetic data is presented. (C) 2014 Elsevier B.V. All rights reserved.