The use of sacrificial agents in photocatalysis is a powerful resource to enhance the performance of photoactive materials. Despite its importance, the effect of the amount of sacrificial agent is not properly described in the literature. In this paper, we have focused on the role of EtOH in the photoreduction of CO2 to CH4 using Cu-P25 photocatalysts in a flow reactor. We found that the production of CH4 increased with the concentration of EtOH, achieving an outstanding CH4 production yield of 235 mu mol/(g & sdot;h) for a flow of 0.25 mu mol/min of EtOH in the gas stream, hinting at the important role of the sacrificial agent in the reaction. The catalytic results together with the characterization of the materials highlight the need to achieve a minimum surface coverage of EtOH on the surface of the catalyst to control the reaction pathway. The adsorption of EtOH is a key factor in boosting the catalytic activity of the best-performing catalyst and producing CH4 from CO2 photoreduction and C2H4O from the photooxidation of EtOH, obtaining two easily separable interesting products for industrial applications in one reaction.