The electrooxidation of formic acid (FA) on Pt has received great attention because of its fundamental significance as a model reaction and its technical importance in fuel cells. Pt layers modified on Au surfaces were recently reported to exhibit enhanced electrocatalytic activity for FA oxidation; however, the mechanistic details have not been clearly elucidated. In this work, the mechanism of FA electrooxidation on Pt layers on Au surfaces was investigated via in situ electrochemical surface-enhanced Raman scattering (SERS). SERS-active DAR@Pt(n) substrates were prepared using the self-terminating electrodeposition of Pt on dendritic Au rod (DAR) surfaces, wherein the amount and coverage of Pt were precisely controlled by applying a different number of potential steps (n) during the electrodeposition process. The electrocatalytic activity of FA was highly dependent on the Pt coverage and thickness on DAR@Pt(n), which was investigated by electrochemical SERS. The amount of CO produced by the dehydration...