In various water-immiscible organic solvents, hemoglobin, myoglobin and cytochrome c deposited on Celite catalyzed the oxidation of dibenzothiophene when peroxides having nonpolar substituents, such as t-butyl hydroperoxide and cumene hydroperoxide (alpha, alpha-dimethylbenzyl hydroperoxide), were used as oxidants. In hexadecane, oxidation of dibenzothiophene by immobilized cytochrome c, with a K-m value of 0.15 mM for dibenzothiophene, was inhibited by cumene hydroperoxide above 0.5 mM.