Hybrid gas/liquid-fed electrochemical flow reactors may become attractive alternatives for chemical synthesis once it is understood how catalytic product selectivity may be optimized through the control of gas phase reactants. Using a constant pH basic electrolyte to suppress the hydrogen evolution reaction, we explore how protonation by water vapor added to the flowing CO2supply affects the CO2reduction reaction. Although H2remains the dominant product, supplying dry CO2gas selectively produces more C2products than C1. However, adding protons through water vapor changes selectivity toward C1products, increasing the overall faradaic efficiency of hydrocarbon production while reducing H2production.