Abstract Catalytic reduction of CO 2 over Cu-based catalysts can produce various carbon-based products such as the critical intermediate CO, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we develop a modified triple-stage quadrupole mass spectrometer to monitor the reduction of CO 2 to CO in the gas phase online. Our experimental observations reveal that the coordinated H 2 O on Cu(I)-based catalysts promotes CO 2 adsorption and reduction to CO, and the resulting efficiencies are two orders of magnitude higher than those without H 2 O. Isotope-labeling studies render compelling evidence that the O atom in produced CO originates from the coordinated H 2 O on catalysts, rather than CO 2 itself. Combining experimental observations and computational calculations with density functional theory, we propose a detailed reaction mechanism of CO 2 reduction to CO over Cu(I)-based catalysts with coordinated H 2 O. This study offers an effective method to reveal the vital roles of H 2 O in promoting metal catalysts to CO 2 reduction.