The catalytic conversion of dibenzothiophenen (DBT) to DBT-5-oxide was examined using microperoxidase-11 (MP-11) as well as a series of heme peroxidases such as horseradish, lignin and manganese peroxidases. MP-11 is a heme-containing oligopeptide obtained by enzymatic hydrolysis of horse heart cytochrome c. Among the catalysts examined, only MP-11 effectively oxidizes DBT to its oxide with an expense of hydrogen peroxide. The maximal oxidation rate was obtained in the aqueous media containing 30% methanol. During MP-11 catalyzed oxidation of DBT to DBT-5-oxide by phenylperacetic acid, phenylacetic acid was formed but not benzyl alcohol. This observation strongly suggested that MP-11 first reacts with peroxide to form compound I species. DBT oxidation by the MP-11/H2O2 system was then carried out either with (H2O2)-O-18 or under O-18(2) atmosphere, indicating that the oxygen atom of DBT-5-oxide was derived from hydrogen peroxide but not from molecular oxygen or water. These results indicated that MP-11 oxidized DBT via the peroxygenation reaction, where the oxygen was transferred from the ferryl-oxy complex of the heme to DBT forming DBT-5-oxide.