Chlorinated copper catalysts have shown promise for electroreduction of carbon dioxide to complex products, but the challenging control of chlorination keeps shaded the potential of chlorine as a selectivity promoter. This work develops a gas-phase chlorination strategy based on exposure to diluted hydrochloric acid at different temperatures to study the effect of chlorine content in copper (II) oxide (CuO), copper (I) oxide (Cu2O), and metallic copper (Cu) foils. Contrary to CuO and Cu, chlorination of Cu2O enhances the formation of highly reduced products (those requiring more than two electron transfers). Faradaic efficiency toward these products (0%–14% at -0.8 V vs. the reversible hydrogen electrode) correlates with the surface chlorine content after reaction (0 to 1.8 atom % chlorine), which is maximized for mild initial chlorination degrees (Cu2O:CuCl∼1). Experimental and computational studies suggest metallic copper surfaces with moderate chlorine coverage and oxychloride-like clusters are active sites responsible for the promotional effect. These findings may facilitate structure-performance relationships, forwarding the next generation of this family of catalysts.