We report on an unexpected domain of high Coulombic efficiency for electrochemical oxidation of aluminum in aqueous polysulfide solutions at high current density for the process : Al + 3OH(-) --> Al(OH)(3) + 3e(-). This high-efficiency domain, of importance to battery processes, includes aluminum oxidation in a wide range of solutions containing concentrated dissolved zerovalent sulfur. As expected at lower concentrations of dissolved sulfur, aluminum electrochemical oxidation is inefficient, due to various exothermic parasitic reactions, including : Al(c) + yS(x)(2-)(aq) + yH(2)O(1) reversible arrow 1/2Al(2)S(3)(c) + yOH(-)(aq) + yHS(-)(aq), y = 1.5/(x - 1), and Al(c) + 1.5S(2)(2-)(aq) + 3H(2)O(1) reversible arrow Al(OH)(3)(amorph) + 3HS(-)(aq). However, at high polysulfide and sulfur concentrations, the Coulombic efficiency can approach 100%. This domain of high efficiency is correlated to an observed cathodic shift with increasing sulfur concentration, leading to improved chemical passivation at the aluminum surface.