[en] An isotopic study of the aqueous-phase oxidation mechanisms of formation of sulfates from SO₂ was made in laboratory experiments by varying the oxygen-18 content (γ¹⁸O) of the SO₂ and the water, and by varying the catalyst, the oxidant, and the order of combination of the SO₂ with air and water. In all of the experiments, the γ¹⁸O of the sulfate product was linearly dependent upon the γ¹⁸O of the water. The results indicate very rapid oxygen-isotope exchange between S(IV) ions and water prior to oxidation to S(VI). Charcoal catalysis of aqueous-phase air oxidation of SO₂ yielded sulfates of significantly lower γ¹⁸O than did Fe³⁺ catalysis. By a different mechanism, aqueous H₂O₂ oxidation of SO₂ yielded sulfates of much lower γ¹⁸O than air oxidation. A comparison of these results with the γ¹⁸O of sulfates found in precipitation water (rain and snow), collected over a three-year period at Argonne, Illinois, indicate that some other mechanism, in addition to aqueous air oxidation, is operative in the atmosphere. (author)