We investigate the influence of electrolyte composition on the electrochemical reduction of CO2 to CO in an electrochemical flow reactor. Specifically, we study the effect of alkali cations on the partial current densities of the two products: CO and H-2. We report that the presence of large cations such as cesium and rubidium in the electrolyte improves the partial current density for CO production. Furthermore, large cations suppress H-2 evolution, resulting in high faradaic yields for CO production. For example, with a large cation, specifically CsOH, a partial current density of 72 mA/cm(2) was obtained at a cathode potential of -1.62 V vs Ag/AgCl. In contrast, in the presence of a small cation, specifically sodium, a partial current density of only 49 mA/cm(2) was achieved at a much more negative cathode potential of -2.37 V vs Ag/AgCl, with NaBr. The effect of cation size on product selectivity for CO production can be explained by the interplay between the level of cation hydration and the extent of cation adsorption on Ag electrodes. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.052301jes] All rights reserved.