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 (E-ox = 2.20 V vs SCE), was oxidized by [(N4Py)Fe-IV(O)](2+) Li in the presence of HClO4 in acetonitrile (MeCN) to yield a stoichiometric amount of benzyl alcohol, in which [(N4Py)Fe-IV(O)](2+) was reduced to [(N4Py)Fe-III(OH2)](3+). The second-order rate constant (k(obs)) 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-d(12) 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)Fe-IV(OH2)](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)Fe-IV(O)](3+) through formation of strong precursor complexes between toluene derivatives and [(N4Py)Fe-IV(OH)](3+).