The reactivity of a nonheme iron(IV)-oxo complex, [(N4Py)Fe IV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl) methylamine), was markedly enhanced by perchloric acid (70% HClO4) in the oxidation of toluene derivatives. Toluene, which has a high one-electron oxidation potential (Eox = 2.20 V vs SCE), was oxidized by [(N4Py)FeIV(O)]2+ in the presence of HClO4 in acetonitrile (MeCN) to yield a stoichiometric amount of benzyl alcohol, in which [(N4Py)FeIV(O)]2+ was reduced to [(N4Py)Fe III(OH2)]3+. The second-order rate constant (kobs) of the oxidation of toluene derivatives by [(N4Py)Fe IV(O)]2+ increased with increasing concentration of HClO4, showing the first-order dependence on [HClO4]. A significant kinetic isotope effect (KIE) was observed when mesitylene was replaced by mesitylene-d12 in the oxidation with [(N4Py)Fe IV(O)]2+ in the absence of HClO4 in MeCN at 298 K. The KIE value drastically decreased from KIE = 31 in the absence of HClO4 to KIE = 1.0 with increasing concentration of HClO4, accompanied by the large acceleration of the oxidation rate. The absence of KIE suggests that electron transfer from a toluene derivative to the protonated iron(IV)-oxo complex ([(N4Py)FeIV(OH)]3+) is the rate-determining step in the acid-promoted oxidation reaction. The detailed kinetic analysis in light of the Marcus theory of electron transfer has revealed that the acid-promoted C-H bond cleavage proceeds via the rate-determining electron transfer from toluene derivatives to [(N4Py)FeIV(OH)] 3+ through formation of strong precursor complexes between toluene derivatives and [(N4Py)FeIV(OH)]3+.