Electrochemical and physical measurements elucidate several thermodynamic properties and chemical factors that affect the performance of a non-aqueous all-vanadium flow battery. An H-type test cell was constructed that demonstrates stable coulombic efficiencies of 70% without flow after several weeks of slow cycling, with a steady plateau voltage near 1.7V during most of the discharge step. Environmental oxygen and water are associated with side reactions that affect long-term charge/discharge response of the battery. Oxygen passivates the electrode and may react with the solvent or supporting electrolyte, while water can cause the formation of oxovanadium complexes. Reversible cycling of the vanadyl acetylacetonate complex appears possible.