Electrocatalytically reducing CO₂ to useful products, such as alcohols and alkaline, attracts people’s interests in designing more efficient electrocatalysts with better selectivity and universality. As this motivation in catalyst modification requires much deeper understanding in the mechanisms of aqueous CO₂ reduction at the heterogeneous interfaces than any learnt knowledge, spectroscopists stand out to utilize in-situ electrochemical spectroscopic techniques to answer these questions. These methods have shed light on considerable questions in the electrocatalysis field. In this system, at a strong optical field in an electrochemical double layer region, how molecules behave with an alternating electromagnetic field and a static electrochemical field is actually the most important question. Among those spectroscopic techniques, vibrational sum-frequency generation spectroscopy (VSFGS) is a recent but powerful tool to be surface-specific and sensitive detecting intermediates during the changes of electrochemical parameters. In this report, potential-dependent behaviors of adsorbed CO from electrocatalytic CO₂ reduction on Au is studied as a prototype for further studies of intermediates in a Cu electrode system.