The aqueous iron(IV) ion, (FeaqO2+)-O-IV, generated from O-3 and Fe-aq(2+), reacts rapidly with various oxygen atom acceptors (sulfoxides, a water-soluble triarylphosphine, and a thiolatocobalt complex). In each case, Fe-aq(IV) O2+ is reduced to Fe-aq(2+), and the substrate is oxidized to a product expected for oxygen atom transfer. Competition methods were used to determine the kinetics of these reactions, some of which have rate constants in excess of 10(7) M-1 s(-1). Oxidation of dimethyl sulfoxide (DMSO) has k = 1.26 x 10(5) M-1 s(-1) and shows no deuterium kinetic isotope effect, k(DMSO-d(6)) = 1.23 x 10(5) M-1 s(-1). The Fe-aq(IV) O2+/sulfoxide reaction is the product-forming step in a very efficient Fe-aq(2+)-catalyzed oxidation of sulfoxides by ozone. This catalytic cycle, combined with labeling experiments in (H2O)-O-18, was used to determine the rate constant for the oxo-group exchange between (FeaqO2+)-O-IV and solvent water under acidic conditions, k(exch) = 1.4 x 10(3) s(-1).