Isotope effect profiles were determined for the deprotonation of a series of para-substituted N-methyl-N(trideuteriomethyl)aniline cation radicals by pyridine and for hydrogen atom abstraction from the corresponding neutral amines by the tert-butoxyl radical. The profiles model reaction steps in two mechanisms commonly proposed for the oxidative dealkylation of amines by cytochrome P-450. Isotope effect profiles were also determined for the P-450 oxidation of the same set of N,N-bis(dideuteriomethyl)anilines by purified CYP2B1, expressed CYP2B1, phenobarbital-induced microsomal P-450, expressed CYP4B1, expressed CYP1A2, and purified CYP102 (BM3). The profiles for all of the P-450 oxidations were found to be experimentally indistinguishable from the hydrogen atom abstraction profile, and distinctly different from the deprotonation profile. This agreement provides strong evidence that the P-450 oxidatively dealkylates the amines by a hydrogen atom abstraction mechanism. Furthermore, the P-450 isotope effect profiles indicate that the reaction mechanism is conserved in both mammalian and bacterial enzymes.