Styrene derivatives are not oxidized by [(N4Py)Fe-IV(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) in acetonitrile at 298 K, whereas epoxidation of styrene derivatives by the iron(IV)-oxo complex occurs efficiently in the presence of triflic acid (HOTf) via proton-coupled electron transfer (PCET) from styrene derivatives to the diprotonated species of [(N4Py)Fe-IV(O)](2+) with HOTf. Logarithms of the firstorder rate constants of HOTf-promoted expoxidation of styrene derivatives with [(N4Py)Fe-IV(O)](2+) and PCET from electron donors to [(N4Py)Fe-IV(O)](2+) in the precursor complexes exhibit a remarkably unified correlation with the driving force of PCET in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes are taken into account. The same PCET driving force dependence is obtained for the first-order rate constants of HOTf-promoted oxygen atom transfer from thioanisols to [(N4Py)Fe-IV(O)](2+) and HOTf-promoted hydrogen atom transfer from toluene derivatives to [(N4Py)Fe-IV(O)](2+) in the precursor complexes. Thus, HOTf-promoted epoxidation of styrene derivatives by [(N4Py)Fe-IV(O)](2+) proceeds via the rate-determining electron transfer from styrene derivatives to the diprotonated species of [(N4Py)Fe-IV(O)](2+), as shown in the reactions of HOTf-promoted oxygen atom transfer from thioanisols to [(N4Py)Fe-IV(O)](2+) and HOTf-promoted hydrogen atom transfer from toluene derivatives to [(N4Py)Fe-IV(O)](2+).