CO₂ electrolysis is a highly useful technique allowing us to upconvert society’s major greenhouse gas into a variety of useful chemicals. However, mechanistically the process is still not totally understood both in 2 electron reduction products (e.g. CO) and in >2e- reduced products (e.g. ethanol/ethylene). This work will discuss using pH variations and deuterium to gain further insight into these mechanisms. One area of debate for 2 electron transfer reduction products such as CO and formate is whether the rate limiting step involves a proton, a water molecule, or is simply due to adsorption of the CO₂ onto the surface. This work will show the benefits of using deuterated water to help resolve this mechanism. By focusing on catalysts geared towards 2 electron transfer reactions to either formate or CO, we have a simple system to break down the mechanism into the reaction order in terms of protons and water molecules. Variations in pH from 3-7 allows us to show the influence of protons for CO or formate, with H₂ evolution on a given catalysts acting as a benchmark. Using deuterated water allows us to denote the influence of water, again using H2 evolution as a benchmark catalyst. Combining both these influences we can mechanistically break down the reactions to denote the mechanism of the rate limiting step. (Since we have obtained relatively straight Tafel slopes over 3 orders of magnitude in current, this entails mass transfer is not significantly influencing our results.) From our results we show that Au and Ag produce a self-consistent mechanism for CO₂ to CO reduction, whereas Sn and In produce a self-consistent mechanism for formate evolution. Pd is unique in that it has the ability to solvate hydrogen and deuterium, thus the combined pH variations and deuterium demonstrates how this effect influences our mechanistic understanding. For greater than 2 electron transfer processes (using Cu) two of the major issues are determining the rate limiting step and the branching point between ethanol and ethylene. We will show how variations in pH and using deuterated water can give insight into both of the aforementioned issues.