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Mechanisms of sulfoxidation catalyzed by high-valent intermediates of heme enzymes have been investigated by direct observation of sulfide-induced reduction of three different compound I species including HRP (horseradish peroxidase), the His64Ser myoglobin (Mb) mutant, and OFe^IVTMP^+• (1) (TMP = 5,10,15,20-tetramesitylporphyrin dianion). The reaction of thioanisole and compound I of HRP (10 μM, pH 7.0, 298 K) gives the resting state of HRP with accumulation of compound II as an intermediate. The yield of sulfoxide by a stoichiometric reaction of HRP compound I with thioanisole was only 25% ± 5%. On the other hand, the same sulfoxidation by both 1 and His64Ser Mb compound I exclusively exhibited a two-electron process, resulting in quantitative formation of sulfoxide. When 1,5-dithiacyclooctane (DTCO) is employed as a substrate, the reaction of His64Ser Mb compound I with DTCO exhibits rapid …