The report gives results of a study of sulfite oxidation, involving the measurement of the rate of enhanced oxygen absorption across an unbroken interface into solution containing sulfite (2-100 mM) and catalyst (0.01-100 mM) at pH 4-6 and 50 C. Fe, Mn, Co, Cu and Cr ions were potent catalysts under these conditions; Ni was inactive. At 10 mM, these catalysts gave pseudo-first order (in oxygen) rate constants of 8.6, 43, 4.7, 95 and 11 sec, respectively. Dry catalyst added in its upper valence state (ferric, cupric, chromic) produced high initial rates that fell in 1 to 6 hours to steady state, while catalyst added in its lower state (ferrous, manganous, cobaltous) showed no high rates and reached steady state in less than 1 minute. Ferric and ferrous eventually resulted in the same rate. Fe was a much stronger catalyst than Mn or Co, but its rate was limited by Fe solubility of about 0.01 mM at pH 5, resulting in an enhancement factor of 2.4 at all higher concentrations. Thiosulfate (0.05-1 mM) had a stronger inhibiting effect and efficiency on Mn than on Fe. EDTA was an effective inhibitor for Fe at equal or greater concentrations. Rates for Fe and Co increased with pH from 4 to 5, while those for Mn were unchanged. Strong positive Mn-Fe synergisms were found to cause absorption rates of up to five times those expected.