The implementation of gas diffusion electrodes is a prerequisite to achieving industrially relevant reaction rates in gas-phase electrochemical CO2 reduction (CO2RR). In the state-of-the-art anion exchange membrane flow electrolyzers, however, there is a substantial loss of reactants due to a nonelectrochemical CO2 consumption at the cathode and the transport of its products to the anode. Our detailed analysis of CO2 crossover in a zero-gap CO2-to-CO flow electrolyzer showed a change in the chemical nature of the transported ionic species through the membrane. With the increasing reaction rate, a continuous shift from HCO3– to CO32– conduction was found to be similar to pure carbonate conduction in the high current density region (>100 mA cm–2). As competing hydrogen evolution takes over the cathodic reaction in a CO2-rich environment, hydroxide conduction becomes more pronounced. This reveals an alteration in the chemical CO2 consumption, the so-called CO2 hydration (CO2 + OH– ↔ HCO3– + OH– ↔ CO32–), implying an unidentical environment for the hydroxide ions generated in CO2RR and hydrogen evolution reaction under a CO2 atmosphere. Our work draws attention to the incomplete description of CO2 hydration at the confined cathode/membrane interface in membrane electrode assembly-type zero-gap CO2 electrolyzers.