The bromine-tetrathionate reaction has been studied in the presence of phosphoric acid/dihydrogen phosphate buffer at T = 25 +/- 0.1 degrees C and at I = 0.5 M ionic strength with both stopped-flow technique and a conventional diode array spectrophotometer. The stoichiometry of the reaction was found to be S4O(6)(2-) + 7Br2 + 10H2O --> 4SO(4)(2-) + 14Br- + 20H+ in bromine excess, but no unambiguous stoichiometry can be established in tetrathionate excess because elementary sulfur as well as hydrogen sulfide are also present in appreciable amounts besides the major product sulfate. It has also been shown that the reaction has two well-separable kinetic phases in an excess of tetrathionate. Rapid disappearance of bromine was observed in the early stage of the reaction followed by a much slower spectral change in the UV region that can be attributed to the disappearance of an absorbing species having much stronger light absorption than that of tetrathionate in the given wavelength range. Two different types of kinetic curves measured by two different instruments have been evaluated simultaneously that led us to suggest and discuss a 10-step model.