The tetrathionate-pypochlorous acid reaction has been investigated in nearly neutral medium at I = 0 5 M ionic strength and T = 25 0 +/- 0.1 degrees C in dihydrogen-phosphate-hydrogen-phosphate buffer by UV-vis spectrophotometry. In excess of hypochlorous acid, the stoichiometry Was found to be S4O62- + 7HOCl + 3H(2)O -> 4SO(4)(2-) + 7Cl(-) + 13H(+). but in excess of tetrathionate colloidal sulfur precipitates. Oil the basis of the simultaneous evaluation of the kinetic curves, a nine-step kinetic model with four fitted and five fixed rate coefficients is proposed. Analogous oxidation reactions Of tetrathionate are also compared and discussed.