We reveal that a significant amount (>30%) of the liquid products from electrochemical CO2 reduction (CO2RR) in a flow reactor can cross the gas diffusion electrode (GDE) and anion exchange membrane (AEM). Crossover of volatile products through the GDE occurs mainly by evaporation, and the migration rate increases linearly with current density and CO2 flow rate. Non-volatile products prefer to cross the AEM through electromigration, and the migration rate is affected by the current density and the flow rate of the catholyte. These results highlight the importance of considering product crossover in CO2RR to adequately assess the catalytic performance. (C) 2020 Elsevier Inc. All rights reserved.