Mechanistic mode for the oxygenation of sulfides with the pig liver microsomal FAD-containing monooxygenase(EC 1.14.13.8) has been conveniently distinguished from that with the phenobarbital induced liver microsomal cytochrome P-450 by analyzing products of the oxygenation of phenacyl phenyl sulfide. Upon oxygenation of phenacyl phenyl sulfide, the FAD-containing monooxygenase gave solely phenacyl phenyl sulfoxide in contrast to the cytochrome P-450 promoted oxygenation which is known to give substantial amounts of C-S bond fission products. The observation suggests that the oxygenation of sulfide with FAD-containing monooxygenase involves the nucleophilic attack of the divalent sulfur on the reactive oxygen atom involved at the enzyme active site, namely electrophilic oxygenation of sulfide, though the oxygenation with the cytochrome P-450 is initiated by a single electron transfer from the sulfide to the enzyme active species.