The effects of electrolyte on selectivity and activity were investigated in the electrochemical reduction of CO2 on the Sn electrode. The production of formate, the primary product in our three-electrode cell was quantitatively characterized by solution phase H-1 NMR spectroscopy. Both SO42- and Na+ favor higher faradaic and energy efficiencies, while HCO3- and K+ enable a higher rate of formate production. The faradaic efficiency was as high as similar to 95% for 0.1 M Na2SO4 at a potential of -1.7 V vs. a Saturated Calomel Electrode (SCE). 0.5MKHCO(3) was an optimal electrolyte for obtaining a high production rate of formate which can reach over 3.8 mu mol min(-1) cm(-2) at a potential of -2.0 V vs. SCE while maintaining a faradaic efficiency of similar to 63%. A trend we observed was that the faradaic efficiency increases as the concentration of electrolyte is diluted. Our studies also show the degradation of electrocatalytic activity of the Sn electrode in this three-electrode cell can be attributed to the electrodeposition of trace amounts of Zn onto the surface of the Sn electrode, which reduces the active surface area. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.049207jes] All rights reserved.