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.