Commercial P25 modified by Au-Cu alloy nanoparticles as thin film exhibits, for CO2 reduction by water under sun simulated light, a rate of methane production above 2000 μmol (g of photocatalyst)(-1) h(-1). Although evolution of hydrogen is observed and O2 and ethane detected, the selectivity of conduction band electrons for methane formation is almost complete, about 97%. This photocatalytic behavior is completely different from that measured for Au/P25 (hydrogen evolution) and Cu/P25 (lower activity, but similar methane selectivity). Characterization by TEM, XPS, and UV-vis spectroscopy shows that Au and Cu are alloyed in the nanoparticles. FT-IR spectroscopy and chemical analysis have allowed one to detect on the photocatalyst surface the presence of CO2(•-), Cu-CO, and elemental C. Accordingly, a mechanism in which the role of Au is to respond under visible light and Cu binds to CO and directs the reduction pathway is proposed.