Chromium acetylacetonate, or Cr(acac)(3), is a promising active species for high-energy-density symmetric redox flow batteries because the neutral complex supports multiple charge-transfer reactions with widely separated redox potentials. Voltammetric and spectroelectrochemical measurements were performed to probe the mechanism of the first electrochemical disproportionation of Cr(acac)(3) - i.e., the cell reaction associated with the two redox couples immediately adjacent to the equilibrium potential of a freshly prepared nonaqueous Cr(acac)(3) solution. Substantially different limiting currents are observed for the positive and negative half-reactions, suggesting that at least one deviates from the similar outer-sphere single-electron transfer mechanisms proposed earlier. Spectroelectrochemical chronoamperometry suggests ligand dissociation in the negative reaction, and consequent structural reorganization of the Cr(acac)(3) complex. Vanadium acetylacetonate was investigated for comparison, and no ligand dissociation was observed. A negative half-reaction mechanism consistent with the voltammetric and spectroelectrochemical data is proposed, and used to rationalize observations of charge/discharge behavior in cycling Cr(acac)(3) cells. (C) The Author(s) 2016. Published by ECS. All rights reserved.