Cytochrome P450 enzymes catalyze three general classes of oxidative reactions : pi-bond epoxidations, heteroatom (N, S, P) oxidations, and carbon hydroxylations. Recent work has shown that cytochrome P450(cam) (CYP101) enantioselectively oxidizes styrene and beta-methylstyrene and that molecular dynamics calculations predict the enantiomeric specificity with remarkable accuracy. Cytochrome P450(cam) is shown here to also catalyze the stereoselective sulfoxidation of thioanisole (R:S 72:28) and p-methylthioanisole (R:S 48:52). Molecular dynamics calculations suggest that oxidations of thioanisole and p-methylthioanisole by cytochrome P450(cam) should yield the corresponding sulfoxides in R:S ratios of 65:35 and 22:78, respectively. The predicted and experimental enantiomer ratios for thioanisole change in a similar manner when a p-methyl substituent is added to the thioanisole. The theoretical treatment correctly predicts the experimental finding that a p-methyl group inverts the preferred sulfoxide stereochemistry.