Two intermediates (2 and 3) are formed consecutively in the reaction of a diiron(III) complex [Fe^III₂(μ-O)(OH)(H₂O)(TPA)₂](ClO₄)₃ (TPA = tris(2-pyridylmethyl)amine, tris(picolyl)amine) with H₂O₂ in CH₃CN at −40 °C. Low-temperature stopped-flow studies showed that both species are kinetically competent in oxidation of phosphines and phenols. The first intermediate (2) reacts with substrates very rapidly (second-order rate constants reach 10⁵–10⁶ M⁻¹ s⁻¹ for substituted triarylphosphines and 10³–10⁵ M⁻¹ s⁻¹ for substituted phenols), in keeping with a diiron(IV)–oxo formulation. The second intermediate (3), a mixed-valent Fe(III)Fe(IV) species, is more stable than 2, and reacts with substrates more slowly (second-order rate constants range from 150 to 550 M⁻¹ s⁻¹ for triaryl phosphine oxidation, and from 18 to 790 M⁻¹ s⁻¹ for phenol oxidation). Reaction rates increase with increasing electron donating abilities of substituents, indicating that both 2 and 3 act as electrophilic oxidants.