The effect of silver on the catalytic activity of Ag/TiO2 and Ag/ZnO systems for the CO2 photoreduction using water vapour as electron donor has been investigated. Catalysts prepared by different impregnation methods and different Ag loadings (1.5% and 3.0 wt.%) were tested in this process. TEM results show that silver is well dispersed as small nanoparticles (2–3 nm) on Ag/TiO2 samples, while it forms larger crystallites (150–300 nm) on Ag/ZnO catalysts. Optical studies reveal that under UV light irradiation silver nanoparticles are reduced to their metallic state, due to the transfer of photoexcited electrons from the TiO2 conduction band. The formation of Ag0 leads to the appearance of surface plasmon resonance (SPR) effect. Regarding the photocatalytic activity, both semiconductors (TiO2 and ZnO) are active towards CO2 photoreduction although titania-based catalysts show higher performances. Further, silver incorporation leads to a significant increase in the hydrocarbon production, mainly attributed to lower electron–hole recombination rates. This may explain the observed enhancement of oligomerization processes, especially favoured by the synergetic effect between Ag nanoparticles and TiO2.