Room-temperature ionic liquids (RTILs) are fascinating electrolytes for CO 2 electroreduction owing to their low overpotentials. Although it is well accepted that the key to understanding the low overpotential is the stabilization of the intermediate, its mechanism is still under discussion. To elucidate the intermediate stabilization mechanism, RTIL/electrode interfaces under CO 2 reduction were observed using surface-enhanced infrared absorption spectroscopy (SEIRAS). This in situ interfaceselective spectroscopy allows us to detect CO 2•− that is not observed in aqueous solutions. For a negative-going potential scan, identical onsets were observed for increasing SEIRA band intensities of CO 2•− and RTIL cations and reduction current. This result indicates that RTIL cations stabilize CO 2 •− on the electrode, and this stabilizing effect can be ascribed to the origin of the low overpotentials of CO 2 electroreduction in RTILs.