The rate of the homogeneous reaction of dissolved oxygen and ammonium sulfite in aqueous solutions without catalysts, which has industrial importance in recovery of sulfur dioxide, was studied at 2O, 25 and 3O °C. A polarographic method using a platinum microelectrode was employed for the determination of the oxygen concentration during the course of reaction. Two reactors of different size, which contained O.5 and 4.7 l of solution, respectively, were found to give essentially the same results. The rate of ammonium sulfite oxidation was found to be about one tenth of that of sodium sulfite. The influence of impurities on the rate was found very remarkable. The existence of inhibitors suggested that the reaction might be a radical one. An empirical rate equation, r=k[SO₃^2-]^3/2[O₂]⁰[H⁺]² in M•min^-1 with k= 1.6× 10³⁹exp(-35× 10³/RT), was obtained in the experimental range of this work.