In this paper, we investigate the effect of temperature on product selectivity for the electrochemical reduction of carbon dioxide (CO2) on polycrystalline copper (Cu). We show that the temperature of an electrolyte solution can influence several reaction parameters in the CO2 reduction reaction (CO2RR), including pH (both local and bulk), concentration of dissolved CO2 ([CO2]), solution resistance, the rate of diffusion of reactants to the electrode surface, and adsorbed intermediates. The working potential was -1.60V vs. Ag/AgCI to allow direct comparison with literature values. Under optimal reaction conditions at 2 degrees C, the faradaic efficiency (FE) for converting CO2 to methane (CH4) on a Cu electrocatalyst increases to ca. 50% while ethylene (C2H4) decreases to ca. 10%. Above room temperature, the production of hydrogen gas (H-2) dominates the electrochemical reaction, reaching >50% FE. The major products (i.e., >5% FE) observed at all temperatures studied were H-2, CH4, C2H4, carbon monoxide, and formate, with product selectivity driven by changes in [CO2] rather than changes in pH. Our initial goal was to confirm pH dependence of CO2RR on Cu at different temperatures. Importantly, we discovered that pH varies minimally over the temperature range studied (2 degrees C to 42 degrees C), in contrast to changes in [CO2] and corresponding changes in product selectivity. (C) 2016 Elsevier B.V. All rights reserved.