Oxidative dimerization of N,N-dimethylaniline (DMA) occurs with a nonheme iron(IV)-oxo complex, [Fe-IV(O)(N4Py)](2+) (N4Py=N,N-bis(2-pyridylmethyl)-N-bis-(2-pyridyl)methylamine), to yield the corresponding dimer, tetramethylbenzidine (TMB), in acetonitrile. The rate of the oxidative dimerization of DMA by [Fe-IV(O)(N4Py)](2+) is markedly enhanced by the presence of scandium triflate, Sc(OTf)(3) (OTf = CF3SO3-), when TMB is further oxidized to the radical cation (TMB center dot+). In contrast, we have observed the oxidative N-demethylation with para-substituted DMA substrates, since the position of the C-C bond formation to yield the dimer is blocked. The rate of the oxidative N-demethylation of para-substituted DMA by [Fe-IV(O)(N4Py)](2+) is also markedly enhanced by the presence of Sc(OTf)(3). In the case of para-substituted DMA derivatives with electron-donating substituents, radical cations of DMA derivatives are initially formed by Sc3+ ion-coupled electron transfer from DMA derivatives to [Fe-IV(O)(N4Py)](2+), giving demethylated products. Binding of Sc3+ to [Fe-IV(O)(N4Py)](2+) enhances the Sc3+ ion-coupled electron transfer from DMA derivatives to [Fe-IV(O)(N4Py)](2+), whereas binding of Sc3+ to DMA derivatives retards the electron-transfer reaction. The complicated kinetics of the Sc3+ ion-coupled electron transfer from DMA derivatives to [Fe-IV(O)(N4Py)](2+) are analyzed by competition between binding of Sc3+ to DMA derivatives and to [Fe-IV(O)(N4Py)](2+). The binding constants of Sc3+ to DMA derivatives increase with the increase of the electron-donating ability of the para-substituent. The rate constants of Sc3+ ion-coupled electron transfer from DMA derivatives to [Fe-IV(O)(N4Py)](2+), which are estimated from the binding constants of Sc3+ to DMA derivatives, agree well with those predicted from the driving force dependence of the rate constants of Sc3+ ion-coupled electron transfer from one-electron reductants to [Fe-IV(O)(N4Py)](2+). Thus, oxidative dimerization of DMA and N-demethylation of para-substituted DMA derivatives proceed via Sc3+ ion-coupled electron transfer from DMA derivatives to [Fe-IV(O)(N4Py)](2+).