Aminium radicals have been proposed as intermediates in amine N-dealkylations and N-oxygenations catalyzed by cytochrome P450 (P450) and some other enzymes. P450s can form some N-oxides and hydroxylamines but these are not favored whenever N-dealkylation is possible. However, if a paradigm involving 1-electron oxidation is valid, then some finite level of partitioning of aminium radicals between N-oxygenation and N-dealkylation might be expected in all cases. Methods were developed for the selective and sensitive measurement of N,N-dialkylaniline N-oxides using high performance liquid chromatography, radiochromatography, and TiCl3 reduction. These N-oxides were relatively stable in the presence of P450 2B1. In the presence of NADPH and NADPH-P450 reductase some reduction to N,N-dialkylamines occurred, along with N-dealkylation (to monoalkylanilines); there was also slow N-dealkylation in the absence of NADPH, which is interpreted in terms of homolytic scission of the N-O bond; N,N-dialkylanilines were not formed nor did the N-oxides support other oxygenation reactions. P450 2B1 (with its reductase and NADPH) formed N-oxides at low rates from several N,N-dialkylaniline derivatives, including N,N-dimethylaniline, N,N-diethylaniline, N-ethyl-N-methylaniline, 4-methyl-N,N-dimethylaniline, 4-cyano-N,N-dimethylaniline, N-phenylpyrrolidine, and N,N-dimethyl-2-aminofluorene. The ratio of N-dealkylation:N-oxygenation varied from 1020 to 6 in this series. These results are consistent with the view that aminium radicals are a branch point in N-oxygenation and N-dealkylation reactions catalyzed by metalloproteins, although some alternate explanations cannot be ruled out. While N-dealkylation is the dominant process in all of the P450-catalyzed amine oxidations, there should be a finite partition ratio between these reactions depending upon the particular enzyme and substrate. The N-oxygenation reaction is probably more complex than a direct radical recombination event and is postulated to involve one or more intermediates.