We have compared the rates of CO formation on Cu and Cu oxide surfaces during the electrochemical reduction of CO2 in aqueous media. On metallic Cu surfaces, H2 formation is the main reaction at potentials less cathodic than –1.16 V(NHE). At this potential the formation of CO becomes significant, while CH4 appears at potentials more cathodic than –1.36 V(NHE). On electrodeposited Cu oxide surfaces there is a complex transient response. During reduction at constant potential (–1.1 V(NHE)), there is a large, transient cathodic current that corresponds to reduction of the oxide layer. After this initial oxide reduction, the current density stabilizes and the formation rates of H2 and CO show a more slowly varying transient behavior. The H2 formation rate is roughly 3x higher than on freshly cleaned Cu foil, but is largely independent of the thickness of the initial oxide layer. In contrast, the CO formation rate is at least one order of magnitude higher on the (reduced) Cu oxide samples than on Cu foil at the same potential. These results are interpreted as evidence that CO formation is enhanced at low-coordination number Cu sites present on freshly nucleated Cu clusters following oxide reduction.