The redox flow battery is a leading candidate to supply society with large-scale energy storage. In the category of aqueous organic redox flow batteries, 9,10-anthraquinone-2,7-disulfonic acid (AQDS) is considered a top-performing molecule and shows ideal electrochemical behavior at low concentrations around 1 mM. However, its behavior at higher concentrations was rarely studied. In this work, the correlation between concentration and electrochemical capacity is evaluated for AQDS in 1 M H2SO4 and 1 M alkaline sodium carbonate buffer, employing cyclic voltammetry, rotating disk electrode voltammetry and diffusion NMR. It was found that an electrochemically inactive dimer was formed by the unreduced AQDS molecules, with dimerization equilibrium constants determined to be K = 5 M−1 for the acidic electrolyte and 8 M−1 for the alkaline one, significantly inhibiting full electrochemical utilization of the system. However, upon reduction of the bulk material, the dimerization will shift in favor of monomer or quinhydrone formation, possibly alleviating the impediment to reduction. Finally, it was found that the dimerization could be decreased slightly by increasing the temperature of the system.