The electrocatalytic reduction of carbon dioxide (CO2) on Au-Ag bimetallic alloy nanoparticles was scrutinized. This system provides a good model to investigate the correlation between the electrocatalytic activity and the composition of the nanoparticles. The CO Faradaic efficiency (and thus the forming CO:H-2 ratio) was changed linearly by controlling the Au:Ag ratio in the alloy. This trend was attributed to the changes in the electronic properties, as there was a monotonous shift in the Fermi level away from the vacuum level with increasing Au content. This shift correspondingly reduces the binding energy of the intermediates (*COOH and *CO), resulting in the gradually enhanced CO2-reduction activity. Finally, comparison of the catalytic activity of Ag@Au core-shell particles to that of pure Ag (which bears similar electronic properties), and to Au-Ag alloy nanoparticles (with similar bulk composition), emphasized the importance of both geometric and electronic effects on the CO2 reduction activity. (C) 2019 Elsevier Ltd. All rights reserved.