The kinetics and the electrochemical promotion of the hydrogenation of CO2 over Ru-catalyst electrodes deposited on yttria-stabilized zirconia (YSZ) and (Na+)-β“-Al2O3 solid electrolytes are investigated at temperatures between 200 and 340 °C and pressures up to 5 bar. In the case of both the O2− conductor (YSZ) and the Na+ conductor (β”-Al2O3), the selectivity for CH4 production is enhanced significantly when using a positive potential by supplying of O2− to, or removal of Na+ from, the catalyst surface. The opposite effect is observed when using a negative applied potential, which suppresses CH4 formation and enhances the production of CO through the reverse water–gas shift reaction. However, at low Na coverage, and under reducing conditions, both methanation and H2 production are promoted by the application of a negative potential. The observed electrochemical promotion behavior in conjunction with the reaction kinetics is consistent with the rules of electrochemical and chemical promotion.