Rates of formation of a non-heme iron(IV)-oxo complex, [Fe-IV(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), via electron-transfer oxidation of [Fe-III(OH)(N4Py)](2+) in acetonitrile (MeCN) containing H2O (0.56 M) were accelerated as much as 390-fold by addition of proton acceptors such as CF3COO-, TsO- (p-MeC6H4SO3-), NsO(-) (o-NO2C6H4SO3-), DNsO(-) (2,4-(NO2)(2)C6H3SO3-), and TfO- (CF3SO3-). The acceleration effect of proton acceptors increases with increasing basicity of the proton acceptors. The one-electron oxidation potential of [Fe-III(OH)(N4Py)](2+) was shifted from 1.24 to 0.96 V vs SCE in the presence of TsO- (10 mM). The electron-transfer oxidation of Fe-III-OH complex was coupled with the deprotonation process by proton acceptors in which deuterium kinetic isotope effects were observed when H2O was replaced by D2O.