Ferrocene-terminated self-assembled monolayers (SAMs) have been studied in the last quarter-century to determine the relationship between their electrochemical properties and microscopic structures as a prototypical chemically modified electrode. Although electronic structures are directly related to the electrochemical properties, knowledge of such a relation concerning ferrocene-terminated SAMs is extremely limited. In this study, we deposited a small amount of ionic liquid onto three types of ferrocene-terminated SAMs possessing different ionization energies and performed X-ray and ultraviolet photoelectron spectroscopies to elucidate the changes in the electronic structure resulting from the adsorption of the ionic liquid.For a low-ionization-energy system, spontaneous oxidation of the ferrocene moieties occurred while their electrochemical activities were retained; for other systems, only minor changes were observed. This result indicates that the interaction between the ionic liquid and ferroceneterminated SAMs facilitates electron transfer when the energy difference between the highestoccupied molecular orbital of the ferrocene moieties and the Fermi level of the electrode becomes small.