AurF and CmlI are currently the only two known diiron arylamine oxygenases. On the basis of extensive quantum mechanical/molecular mechanical (QM/MM) spectroscopic and mechanistic modelings, here we predict that the key oxygenated intermediates in AurF and CmlI, so-called P, are uniformly hydroperoxo species having similar structures. As a basis for mechanistic unification in AurF and CmlI, the proposed diferric-hydroperoxo P is calculated to be able to promote the arylamine N-oxygenation with highly accessible kinetics. This convergent μ-η0:η2 structural assignment of P's in AurF and CmlI can rationalize many conundrums for P, including the different Mössbauer spectroscopic parameters, low O-O vibrational frequency, ambiphilic reactivity, and inertness toward C-H activation. In view of the very limited knowledge about hydroperoxo species in diiron enzymes, the novel diferric-hydroperoxo-mediated N-oxygenation mechanism revealed in this work opens up a new avenue for understanding the O2 activation mode in nature. For elucidating the structures of transient oxidants for diiron enzymes, the promising approach of QM/MM Mössbauer spectroscopic modeling is highlighted as a key problem solver in mechanistic enzymatic research.