We have carried out species determination and kinetic analysis in the hydrogen peroxide-thiosulfate reaction by means of capillary electrophoresis and high performance liquid chromatography. In addition to thiosulfate, dithionate, trithionate, and tetrathionate, other polythionates such as pentathionate and hexathionate were detected during the oxidation process. The polythionates found are sensitive to the pH, with the average length of the sulfur chain decreasing with increasing pH. By varying the pH and the concentrations of the reactants, we find that the reaction is first order with respect to each of the reactants with rate constant k = 0.025 M-1 s(-1). With HOS2O3-, HSO3-/SO32-, S3O62-, S4O62-, and S5O62- as key intermediates that are eventually oxidized to sulfate, a proposed 14-step kinetic model simulates the reaction process, including the evolution of the thiosulfate and tetrathionate concentrations at various pHs.