The metabolism of three arylamine substrates by H2O2 in the presence of each of the peroxidative enzymes chloroperoxidase (CPX) and pea seed peroxygenase (PSM) was conducted with normal H2O2 and with 18O-labeled H2O2. The resulting C-nitroso aromatic metabolites were examined by GC-MS methods to determine the extent of 18O incorporation. The arylamine substrates were p-toluidine, 4-chloroaniline, and 3,4-dichloroaniline. For both enzymes, all three arylamines were found to give quantitative incorporation of 18O into their nitroso metabolites when [18O]H2O2 was the oxidant substrate. The introduction of the oxygen atom into 4-chloronitrosobenzene was found to occur during the first step of this process, since it was found that when (4-chlorophenyl)hydroxylamine was employed as the substrate, no significant incorporation of 18O occurred. These observations prove that CPX and PSM cause N-oxidation of primary arylamines via an oxygen transfer from the compound I activated forms of their heme functional groups. Therefore, these peroxidases are correctly called peroxygenases when acting in such a manner. A discussion of the reaction mechanisms for peroxidases and their relation to cytochrome P-450 oxidations is presented.