We’ve combined experimental and computational techniques to characterize the catalytic activity of 2-3 nm, bimetallic gold-copper nanoparticles (NPs) for CO2RR. The catalysts showed composition-dependent CO2RR activity that exceeded both the parent gold and copper metals. Interestingly, the bimetallic NPs produced only CO and H2, whereas bulk, polycrystalline copper also produced methane and ethylene. Electrocatalytic activity studies identified 21% Cu NPs as the most active composition with reaction turnover (TOF) frequencies exceeding 60 s-1 and Faradaic Efficiencies approaching 100% FE at -1.0V vs. RHE. The enhanced activity indicates a synergistic effect upon diluting gold atoms in copper. Laboratory and synchrotron-based X-ray spectroscopies were coupled with density functional theory (DFT) to confirm Au-Cu charge transfer within the NPs and identify mixing between near Fermi-level electronic states. We found a strong correlation between the experimentally-determined TOFs and the computationally-predicted binding energy of key reaction products. The superior performance of 21% Cu NPs stemmed from facile desorption of reaction products and extremely weak H binding. This study shows that atomic composition can be used to tune the electronic structure and chemical reactivity of small bimetallic NPs.