Flow electrolyzers are attracting significant attention because of their unique capability of facilitating carbon dioxide (CO₂) electroreduction at high reaction rates. Among all figures of merit, CO₂ single-pass conversion is an important factor that can strongly affect the product separation cost of the whole process but often neglected in the literature. In this study, CO₂ single-pass conversion was investigated using a flow electrolyzer under various operating conditions to identify the operating constraints on achieving a maximum single-pass CO₂ conversion. The maximum amount of CO₂ being converted to CO is limited to approximately 43% regardless of CO₂ feeding rate, operating current density, and reaction temperature. Further investigation shows that the remaining CO₂ feed was mainly consumed by the side reaction of carbonate formation between the CO₂ feed and the hydroxide anions generated during the electrolysis. As a result, the gas effluent stream from the cathode chamber contains mainly CO (∼80%), together with 15% H₂ and 5% unreacted CO₂.