The oxidation of cobalt(II) complexes in tris(hidroxymethyl)-aminomethane (TRIS) buffers in aqueous medium, by dissolved oxygen, is strongly accelerated by sulfur(IV). The process is still faster in the presence of Mn2+. The reaction is adequate for S(IV) determination as it is stabilized by the amine and provides an analytical curve, with a slope as high as 7.4 × 103L.mol−1.cm−1at 230 nm referred to the concentration of cobalt(III) formed, proportional to the sulfite concentration. The method is being used for S(IV) determination in sulfur hexafluoride aged under electrical stress. It presents the advantage of avoiding the use of toxic sodium azide from the former procedure.