Benzophenone and 1,4-di-tert-butyl-2,5-dimethoxybenzene are used as the anode and cathode active species respectively in an all-organic redox-flow battery. A number of organics as the co-solvents are applied in the electrolyte to improve the electrochemical performance of it. For all kinds of the mixed solvents, a lower content of acetonitrile leads to a higher solubility to 1,4-di-tert-butyl-2,5-dimethoxybenzene and a lower conductivity. The results of cyclic voltammetry tests demonstrate that the electrode reactions are controlled by diffusion. With the decrease of the content of acetonitrile, the dynamic viscosity of the electrolyte increases, which generally leads to the decrease of the diffusion coefficients of the active species.