The electrochemical reduction of carbon dioxide to fuels and high-value commodity chemicals is of great technological interest. Cu is the only pure electrocatalyst that facilitates the conversion of carbon dioxide to hydrocarbons at significant reaction rates. Cu is therefore an important prototypical catalyst for the reduction of carbon dioxide. However, the product selectivity of this process for specific desired products is poor. This product selectivity is not only controlled by the electrode material and its morphology, but also by the cations of the supporting electrolyte. While it has been shown that the identity and concentration of cations can greatly change the reaction selectivity of the process, a fundamental understanding of these effects is mostly lacking to date. Herein, we systematically explored the effects of quaternary alkyl ammonium ions on the reduction of CO to ethylene on Cu electrodes. With differential electrochemical mass spectrometry (DEMS), we determined that ethylene is produced in the presence of methyl4N+ and ethyl4N+. By contrast, we did not observe the formation of this product in the presence of propyl4N+ and butyl4N+. Using surface-enhanced infrared absorption spectroscopy (SEIRAS), we determined that the changes in selectivity are likely not due to blocking of catalytic sites or changes in the interfacial electric field. However, SEIRAS showed that the interaction of surface-adsorbed CO and water is disrupted in the presence of the two larger cations. This finding indicates that the interaction of interfacial water with surface-adsorbed CO is critical for the formation of ethylene. We will discuss the mechanistic implications of these findings. Furthermore, we will discuss the effects of alkali cations on the hydrogen evolution reaction, which is an undesired side-reaction in electrochemical carbon dioxide reduction. Using electroanalytical techniques, we showed that the effects are strongly dependent on the pH of the electrolyte. We rationalize these observation with dependence of the reaction mechanism of the hydrogen evolution reaction on pH, and the resulting differential interactions of cations with the intermediates in the different pH regimes.