Studies on the sulfoxidation of aryl alkyl sulfides by purified toluene dioxygenase and naphthalene dioxygenase showed that naphthalene dioxygenase produces sulfoxides of (S) absolute configuration in high enantiomeric purity while those formed by toluene dioxygenase are of variable enantiomeric purity depending on the p-substituents on the benzene ring. Oxygen uptake experiments with naphthalene dioxygenase showed that the reaction rate and degree of oxygen incorporation are affected by both aryl and alkyl substituents. 18O2-Incorporation experiments showed that the oxygen atom of methyl phenyl sulfoxide formed by toluene dioxygenase and naphthalene dioxygenase is derived exclusively from O2. Accompanying studies showed that chloroperoxidase produces single (R)-sulfoxides (> 98% enantiomeric excess) from the aryl alkyl sulfides examined in the present study.