Oxoiron(IV) is a common catalytic byproduct observed in the oxidation of alkenes by the combination of H2O2 and nonheme iron catalysts including complex 1, (FePDP)-P-II* (where PDP* = bis(3,5-dimethy1-4-methoxypyridy1-2-methyl)- (R,R)-2,2' -bi-pyrrolidine). The oxoiron(IV) species have been proposed to arise by O-O homolysis of the peroxyiron(III) or acylperoxyiron(III) intermediates formed during the presumed Fe-III-Fe-V catalytic cycle and have generally been regarded as off-pathway. We generated complex 1(IV)=O (lambda(max) = 730 nm, epsilon = 350 M-1 cm(-1)) directly from 1 and an oxygen atom donor IBXi-Pr (isopropyl 2-iodoxybenzoate) in acetonitrile in the temperature range from -35 to +25 degrees C under stopped-flow conditions. Species 1(IV)=O is metastable (half-life of 2.0 min at +25 degrees C), and its decay is accelerated in the presence of organic substrates such as thioanisole, alkenes, benzene, and cyclohexane. The reaction with cyclohexane-d(12) is significantly slower (KIE = 4.9 +/- 0.4), suggesting that a hydrogen atom transfer to 1(IV)=O is the rate limiting step. With benzene-d(6), no significant isotope effect is observed (KIE = 1.0 +/- 0.2), but UV-vis spectra show the concomitant formation of an intense 580 nm band likely due to the Fe(III)-phenolate chromophore, suggesting an electrophilic attack of 1(IV)=O on the aromatic system of benzene. Treatment of 1(IV)=O with H2O2 resulted in rapid decay of its 730 nm visible band (k = 102.6 +/- 4.6 M-1 s(-1) at -20 degrees C), most likely occurring by a hydrogen atom transfer from H2O2. In the presence of excess H2O2, the oxoiron(IV) is transformed into peroxyiron(III), as seen from the formation of a characteristic 550 nm visible band and g(eff) = 2.22, 2.16, and 1.96 electron paramagnetic resonance (EPR) spectroscopy signals. Reductively formed 1(III)-OOH was able to re-enter the catalytic cycle of alkene epoxidation by H2O2 "albeit with lower yields versus those of oxidatively formed (i.e., 1 + H2O2) peroxyiron(III) owing to a loss of ca. 40% active iron. As such, the oxoiron(IV) species can be reintroduced to the catalytic cycle with extra H2O2, acting as an iron reservoir. Alternatively, peroxycarboxylic acids, which have a stronger O-H bond dissociation energy, do not reduce 1(IV)=O, ensuring that more oxidant is productively employed in substrate oxidation. While this reaction with H2O2 may occur for other nonheme oxoiron(IV) complexes, the only previously reported examples are 3(IV)=O and 4(IV)=O, which are reduced by hydrogen peroxide 130- and 2900-fold more slowy, respectively (as in Angew. Chemie - Int. Ed. 2012, 51 (22), 5376-5380, DOI: 10.1002/anie.201200901).