The possibility that adsorbed carbon monoxide may act as a reaction intermediate rather than merely a catalytic poison for formic acid electrooxidation on Pt-group metals in aqueous perchloric acid is explored by monitoring the time-dependent effects of reactant 13C/12C isotopic substitution on the adsorbate vibrational properties, as discerned from surface-enhanced Raman spectroscopy (SERS). The electrodes examined, polycrystalline rhodium and iridium (deposited as ultrathin films on gold to yield optimal SERS activity), exhibit substantial formation of adsorbed CO at potentials below and close to the onset of formic acid electrooxidation, as discerned from the well-known C−O (νCO) stretches at 1850−2000 cm-1 and the metal−CO vibrations at 450−500 cm-1. As in earlier infrared studies, the former vibrations provide a sensitive means of monitoring adsorbate 13CO/12CO replacement from the characteristic ∼45 cm-1 difference in isotopic νCO frequencies. These SER vibrational features are used to monitor the r...