Salt-ion crossover through bipolar membranes (BPMs) is a major source of capacity fade and efficiency loss in acid-base flow batteries (ABFBs), yet its mechanisms remain poorly understood. Here, we quantify Na+ and Cl− crossover in a five-cell ABFB stack under varying electrolyte composition, state of charge (SoC), current density, and temperature. The results show that crossover occurs predominantly through BPMs and is governed by diffusion assisted by H+/OH− neutralization. At ambient conditions, Cl− crossover exceeds Na+ by roughly twofold, with apparent activation energies of 15 kJ mol−1 (Cl−) and 33 kJ mol−1 (Na+), reflecting asymmetric co-ion diffusion within the cation- and anion-exchange layers. Increasing current density reduces the relative contribution of crossover, whereas higher electrolyte concentration, SoC, and temperature increase it. Based on the combined voltage efficiency and crossover analysis, we recommend operating ABFBs under conditions corresponding to BPM efficiencies above 80% - that is, with dilute electrolytes (≤0.25 M), near-equimolar acid and base (SoC ≈ 50%), and cycling current densities exceeding 5 mA cm−2. These insights clarify the interplay between salt-ion transport and BPM operation, and provide design guidelines for next-generation ABFBs and other BPM-based electrochemical systems.