Abstract The effect of different positive supporting electrolytes on the performance of a bench-scale Zn-Ce redox flow battery (RFB) has been studied. The effectiveness of mixed methanesulfonic/sulfuric acid, mixed methanesulfonic/nitric acid, and pure methanesulfonic acid has been assessed and compared on the basis of the cyclic voltammetric response for the Ce(III)/Ce(IV) redox couple and galvanic charge-discharge of a bench-scale Zn-Ce RFB. The Ce(III)/Ce(IV) reaction exhibits faster kinetics and the RFB exhibits higher coulombic efficiency and long-term performance over ~40 charge-discharge cycles in the mixed 2 mol/L MSA–0.5 mol/L H₂SO₄ electrolyte compared to that achieved in the commonly used 4 mol/L MSA electrolyte due to lower H⁺ crossover and higher Ce(IV) solubility. The coulombic efficiency fade rate in the mixed MSA-H₂SO₄ electrolyte is 0.55% per cycle over 40 charge-discharge cycles, while the fade rate is 1.26% in the case of 4 mol/L MSA. Furthermore, the positive electrode reaction is no longer the limiting half-cell reaction even at the end of long-term battery charge-discharge operation. This work shows that a mixed MSA–H₂SO₄ acid electrolyte may be a better option for the positive side of a Zn-Ce RFB as a large-scale energy storage device.