Electroreflectance (ER) and transmission -absorption spectral features of viologen cation radical moieties in close proximity of Au nanoparticles were investigated at electrified interfaces. ER spectra were measured at a polycrystalline Au electrode coated with a thiol-functionalized alkyl viologen (viologen-thiol: VT) monolayer on which Au nanoparticles (11 nm diameter) are immobilized. Transmission-absorption spectra were measured for VT adsorbed on Au nanoparticles immobilized on a mercaptosiloxane-modified ITO electrode. At the Au electrode, an anomalous ER spectrum was observed around the formal potential of viologen, while an ER signal due to the charging-discharging of the Au particles was observed at more positive potentials. At the ITO electrode, both potential modulated absorption spectra and difference absorption ones with respect to potential showed the anomaly as the same as the ER spectrum at the Au electrode. The anomaly is that the absorption spectrum is not a simple sum of the spectra of Au nanoparticles and viologen cation radicals, suggesting 4 strong interaction between viologen cation radical moieties and Au particles under plasmon excitation of the particles.