The reaction of [FeII(tris(2-pyridylmethyl)amine, TPA)(NCCH3)2]2+ with 1 equiv. peracetic acid in CH3CN at -40°C results in the nearly quantitative formation of a pale green intermediate with γmax at 724 nm (ε ≈ 300 M-1·cm-1) formulated as [FeIV(O)(TPA)]2+ by a combination of spectroscopic techniques. Its electrospray mass spectrum shows a prominent feature at m/z 461, corresponding to the [FeIV(O)(TPA)(ClO4)]+ ion. The Mössbauer spectra recorded in zero field reveal a doublet with ΔEQ = 0.92(2) mm/s and δ = 0.01(2) mm/s; analysis of spectra obtained in strong magnetic fields yields parameters characteristic of S = 1 FeIV=O complexes. The presence of an FeIV=O unit is also indicated in its Fe K-edge x-ray absorption spectrum by an intense 1-s → 3-d transition and the requirement for an O/N scatterer at 1.67 Å to fit the extended x-ray absorption fine structure region. The [FeIV(O)(TPA)]2+intermediate is stable at -40°C for several days but decays quantitatively on warming to [Fe2(μ-O)(μ-OAc)(TPA)2]3+. Addition of thioanisole or cyclooctene at -40°C results in the formation of thioanisole oxide (100% yield) or cyclooctene oxide (30% yield), respectively; thus [FeIV(O)(TPA)]2+ is an effective oxygen-atom transfer agent. It is proposed that the FeIV=O species derives from O-O bond heterolysis of an unobserved FeII(TPA)-acyl peroxide complex. The characterization of [FeIV(O)(TPA)]2+ as having a reactive terminal FeIV=O unit in a nonheme ligand environment lends credence to the proposed participation of analogous species in the oxygen activation mechanisms of many mononuclear nonheme iron enzymes.