Aqueous organic flow batteries (AOFBs) are promising energy storage devices featuring low cost, easy scale-up, and improved safety. Extensive research effort has been devoted to exploring redox-active organic compounds, whereas another critical component, the ion-conducting membrane, is overlooked. Here, we demonstrate the syntheses of an anion-conducting membrane and a cation-conducting membrane from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with slightly different procedures. We evaluate the performance of PPO-based membranes in both neutral and alkaline AOFBs. The cells delivered a Coulombic efficiency of similar to 100%, a capacity retention rate of up to 99.988% per cycle (or 99.800% per hour), and an energy efficiency of 81% over 800 consecutive cycles. The permeability of electrolytes across the membrane was less than 4.80 x 10(-9) cm(2) s(-1). The membranes are robust and stable and can be manufactured at approximately $9 per square meter. Our results suggest they are promising membrane candidates for AOFBs.