Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR–SEIRAS (attenuated total reflectance–surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm–2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is −0.25VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until −0.5VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3– wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3– bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.