As the demand for sustainable energy grows, large-scale energy storage solutions are becoming in- creasingly essential to balance power supply and demand, ensuring grid stability and security. Acid- base flow batteries (ABFBs) have emerged as a promising energy storage solution due to their cost- effective electrolytes, high energy density, and environmental advantages. However, co-ion crossover in ABFBs remains a critical challenge, leading to pH imbalances, capacity fade, and reduced efficiency over cycling. In this study, the mechanistic aspects of co-ion transport in ABFBs are investigated, and their impact on battery performance is evaluated. Experiments are conducted under varying electrolyte concentrations, state of charge, current densities, and temperatures to quantify crossover ion flux and correlate it with key performance metrics, including round-trip efficiency, power density, and capacity fade rate. The results provide insight into the co-ion crossover mechanisms and their influence on the cycling stability of the battery. These findings contribute to the optimization of ABFBs by identifying operational strategies to mitigate co-ion crossover and enhance overall performance, advancing the development of high-efficiency ABFB systems. Keywords: Acid-base flow batteries, co-ion crossover, cycling stability, ion transport phenomena, ca- pacity fade, bipolar membranes, ion exchange membranes