The reaction of NO-Fe(TPP) with low pressures of NO gas proceeds through three distinct transformations, the first of which we suggest is the formation of an N-N-coupled, (NO)(2) adduct intermediate. The subsequent formation of NO(NO2)Fe(TPP), which under these conditions readily loses NO, suggests that it is formed by addition of free NO2 to the starting nitrosyl, A mechanism is proposed which implies that the addition of a competitive O atom acceptor would lead to catalytic production of N2O. In agreement with the proposed mechanism, the formation of N2O is decoupled from the formation of the nitrite by using PPh3 as the competitive acceptor. The mechanism of O atom transfer was examined by cross-labeling experiments, which show that both O atoms in the intermediate are equivalent, even under catalytic conditions. The formation of an intermediate was confirmed by IR spectroscopy of the heterogeneous reaction of an NO:Fe(TPP) film with gaseous NO, in which transient, isotope-sensitive nu (NO) bands are seen prior to NO(NO2)Fe(TPP) formation. Mixed N-14/N-15 label experiments demonstrate coupling between the two bound nitrosyls in the transient species.