Flow batteries for grid scale energy storage have been proposed as a technological solution to the intermittency of renewable energy sources. Aqueous-soluble organic reactant molecules composed of earth abundant elements, such as quinones, show promise for reducing system cost. In addition to low cost, sufficient capacity retention is also a prerequisite for the practical success of any flow battery chemistry. For inorganic systems, capacity retention is often determined by crossover of reactant species, whereas organic reactant crossover is typically lower due to a larger molecular size. Instead of crossover, reactant molecule stability can become the limiting factor for organic flow battery lifetime. In an operating battery, the lifetime of the reactant molecules depends on their chemical stability within the electrolyte as well as on their electrochemical stability during reduction and oxidation. We report here some of the results pertaining to the chemical and electrochemical stability of several organic and organometallic redox reactants in aqueous electrolytes. Chemical stability of molecules is monitored by detection of potential decomposition products by NMR spectroscopy during long duration storage at room temperature and at 60 °C. Electrochemical stability is determined by monitoring oxidation and reduction peak currents during standard three-electrode cyclic voltammetry in addition to capacity measurements in a custom designed two-electrode cell.