Carbon dioxide (CO₂) reduction flow cells are a promising means of curtailing anthropogenic CO₂ emissions by reducing CO₂ to generate useful carbon fuels when coupled with renewable energy sources. However, operating these devices at commercially relevant conditions is typically accompanied by mass transport issues related to unstable performance that is often overlooked. This thesis contains two studies focused on identifying and characterizing the source of unstable performance in these devices. In the first study, the effect of gaseous accumulation at the cathode gas diffusion electrode interface on the performance of an alkaline flow cell was investigated. In the second study, the relationship between unstable operation and the electrolyte layer gas saturation in a flow cell was characterized. These two studies aim to provide the necessary first steps in addressing unstable operation in CO₂ reduction flow cells towards achieving stable, commercially relevant operation.