The present paper first reports a kind of supporting electrolyte, mixed-acid media (CH3SO3H and H2SO4), used in redox flow battery (RFS) technology. Experimental work is performed with the aim of evaluating the Ce3+/Ce4+ redox couple in mixed-acid electrolyte for use in RFB technology. The mixed-acid media have their special advantages: (i) the Ce3+/Ce4+ electrode reaction appears to be more reversible in mixed-acid solution; the peak splittings in mixed acid are significantly less than in sulfuric acid; the diffusion coeffcient in mixed acid is larger than in single acid (CH3SO3H or H2SO4); the exchange current density and standard rate constant of the Ce3+/Ce4+ redox reaction in mixed-acid media on graphite electrode are 8.05 x 10(-3) A cm(-2) and 4.17 x 10(-4) cm s(-1); (ii) the solubility of cerium salt in mixed acid is larger than in sulfuric acid; a solution of 1 mol dm(-3) cerium containing 2 mol dm(-3) MSA and 0.5 mol dm(-3) H2SO4 is sufficiently stable for more than 1 month at temperatures up to 313 K; (iii) Coulombic and energy efficiencies of the test cell using mixed-acid electrolyte are both higher than that using single CH3SO3H electrolyte; the average Coulombic and energy efficiencies of the cell using mixed acid are 87.1 and 73.5%, respectively, which are comparable to that of the all-vanadium RFB.