High-valent iron-oxo intermediates have been identified as the key oxidants in the catalytic cycles of many nonheme enzymes. Among the large number of synthetic Fe-IV=O complexes characterized to date, [Fe-IV(O)(N4Py)](2+) (1) exhibits the unique combination of thermodynamic stability, allowing its structural characterization by X-ray crystallography, and oxidative reactivity sufficient to cleave C-H bonds as strong as those in cyclohexane (DC-H = 99.3 kcal mol(-1)). However, its redox properties are not yet well understood. In this work, the effect of protons on the redox properties of 1 has been investigated electrochemically in nonaqueous and aqueous solutions. While the cyclic voltammetry of 1 in CH3CN is complicated by coupling of several chemical and redox processes, the Fe-IV/III couple is reversible in aqueous solution with E-1/2 = +0.41 V versus SCE at pH 4 and involves the transfer of one electron and one proton to give the Fe-III-OH species. This is in fact the first example of reversible electrochemistry to be observed for this family of nonheme oxoiron (IV) complexes. C-H bond oxidations by 1 have been studied in H2O and found to have reaction rates that depend on the C-H bond strength but not on the solvent. Furthermore, our electrochemical results have allowed a DO-H value of 78(2) kcal mol(-1) to be calculated for the Fe-III-OH unit derived from 1. Interestingly, although this DO-H value is 6-11 kcal mol(-1) lower than those corresponding to oxidants such as [Fe-IV(O)(TMP)] (TMP = tetramesitylporphinate), [Ru-IV(O)(bpy)(2)(py)](2+) (bpy = bipyridine, py = pyridine), and the tert-butylperoxyl radical, the oxidation of dihydroanthracene by 1 occurs at a rate comparable to rates for these other oxidants. This comparison suggests that the nonheme N4Py ligand environment confers a kinetic advantage over the others that enhances the C-H bond cleavage ability of 1.