Flat model and powder Cu, ZnO/Cu, and CeOx/Cu catalysts were studied by focusing on the role of the oxide phase as a promoter in the water gas shift (WGS) and its reverse reaction (RWGS). Activity measurements of the powder catalysts showed that both oxides enhance Cu reactivity, with CeOx/Cu being more active than ZnO/Cu in the WGS reaction. In situ ultraviolet−visible spectroscopy, exploiting the localized surface plasmon resonances of metallic Cu nanoparticles, together with X-ray photoelectron spectroscopy was then used to elucidate the origin of the enhanced reactivity on flat model catalysts. These experiments showed that ZnO and CeOx promote H2O and CO2 dissociation, leading to oxidation of the Cu nanoparticles. CeOx performs better in this respect than ZnO. This is important because the reactivity in the WGS and RWGS reactions is related to the ability to activate H2O and CO2. The Ce3+ ions are identified as the most efficient sites for H2O and CO2 dissociation, while Cu0 keeps Ce3+ stable by promoting reduction of Ce4+ during the dissociation process. In this sense, the CeOx/Cu catalyst forms a bifunctional catalyst, which is more active in the (R)WGS than CeOx and Cu catalysts separately.