We used vibrational sum frequency generation spectroscopy (SFG) synchronized with cyclic voltammetry (CV) to study CO2 electroreduction on polycrystalline Ag, in a room-temperature ionic liquid (RTIL) electrolyte, 1-ethyl-3-methylimidazolium tetrafluorborate (EMIM-BF4). CO was produced from CO2 at - 1.33V (vs. Ag/AgCl). SFG measurements showed CO adsorbed on Ag, but at - 1.33V adsorption was weak, so CO did not poison Ag. At this same - 1.33 V potential, the nonresonant SFG intensity was a minimum and the curvature of the SFG intensity vs. potential relation changed dramatically. This was true whether or not CO2 was present. The SFG results indicate the RTIL undergoes a potential-driven structural transition in the double layer, and this transition controls the reduction of CO2. Adding water to the RTIL increases the efficiency of CO2 reduction and shifted the structural transition 80 mV to lower potential.