Cu O/gas diffusion layer (GDL) electrodes prepared by electrodeposition were studied for the electrocatalytic reduction of CO . The designed electrode was also tested in solar-light-induced CO conversion in combination with a CuO/NtTiO photoanode using a compact photoelectrocatalytic (PEC) cell. Both PEC cell electrodes were prepared using non-critical raw materials and low cost, easily scalable procedures. In the PEC experiments, a total carbon faradaic selectivity of about 90 % to formate and about 75 % to acetate was obtained after 24 h of operations without application of potential/current or using sacrificial agents. In electrocatalytic tests of CO reduction at -1.5 V, the same electrode yielded high total faradaic selectivity (>95 %) but formed selectively formate (about 80 % selectivity) rather than acetate. The in situ transformation of the Cu O/GDL electrode leads to the formation of a hybrid Cu O-Cu/GDL system. Cyclic voltammetry data indicate that the potential and the presence of CO (not only of HCO species) are both important elements in this transformation. Data also indicate that the surface concentration of CO (or of its products of transformation) on the electrode is an important factor to determine performance in the conversion of CO .