Reaction pathways for oxidation of ammonia and mono-, di-, and trimethylamine by singlet and triplet oxygen atoms as models for cytochrome P450 enzymatic oxidation have been characterized by using the semiempirical molecular orbital method MNDO. Enthalpies and entropiesof formation have been calculated for reactants, transition states, intermediates, and products on closed-shell and triplet pathways, and free energies of reaction and activation have been calculated from them. Energy minima and transition states have been verified by calculation of force constants. The results indicate a two-step, addition—rearrangement mechanism for nonradical oxidation leading to both N-hydroxy and N-methoxy products via N-oxide intermediates. While barriers to the rearrangement are higher than to N-oxide formation, the first step is determining the overall reaction in the gas phase. On a triplet surface, both α-C- and N-oxidation are competitive. N-Oxidation via an addition mechanism appears to be favored over an H-abstraction mechanism. However, in contrast to a closed-shell mechanism, no stable N-oxide radical intermediate is found, and the barrier to formation of N-hydroxy and N-methoxyl products on a triplet surface is greater. Additional gas phase, solution, and enzymatic studies, particularly focusing on identification of transient intermediates and products, are necessary to further distinguish among these mechanisms.