Cu electrodes of high surface roughness are often found the covert CO and CO₂ at high rates and good product selectivity to C₂₊ hydrocarbons in comparison with their smooth counterparts. The origins of their desirable catalytic properties are still controversial. Their catalytic characteristics have been variably attributed to the prevalence of certain facets, a suitable surface density of step sites, the presence of metastable oxide phases, local pH effects, and other factors. While the catalytic activity likely arises from a combination of these reasons, the atomic-level morphology of these rough electrodes certainly plays a critical role. However, very few techniques are available for probing the atomic-level morphology of rough electrodes under electrochemical conditions. Further, the methods that can probe the surface in situ often require experimental conditions that are quite distinct from those employed in typical product detection studies. To address these challenges, we introduce here a methodology that permits the identification of the prevalent surface facets on rough metal electrodes under product detection conditions. Our method combines surface-enhanced infrared absorption spectroscopy (SEIRAS) with differential electrochemical mass spectrometry (DEMS). Specifically, we compared the CO reduction activity of two types of SEIRAS-active thin film electrodes. Cu films that are electrochemically deposited on Si-supported Au films (CuAu-Si) exhibit an onset potential for ethylene that is ∼200 ± 65 mV more cathodic than Cu films that are electrolessly deposited onto Si crystals (Cu-Si). By monitoring the lineshape of the CO stretch band of surface-adsorbed CO as a function of applied potential, we show that the two types of films exhibit different prevalent surface facets. Our analysis reveals that CuAu-Si is rich in (111) facets, whereas Cu-Si is rich in (100) facets. The different predominant surface facets on these two rough electrodes explain their different catalytic activities for ethylene formation. Our work establishes the potential-dependence of the lineshape of the CO stretch band of surface-adsorbed CO as a probe of the atomic-level surface morphology of rough metal electrodes. When this approach is combined with complementary techniques, this methodology provides essential structural information for further improvement in the product selectivity of rough metal electrodes.