Abstract: An amorphous solid composition for a vanadium redox flow battery electrolyte, which can be suitably used for storage of excess electric power for using in daytime, generated in nighttime by a power station, storage of electric power generated by photovoltaic power generation or wind power generation, and the like. The amorphous solid composition for vanadium flow battery electrolyte is characterized in that the weight ratio of the vanadium content in the tetravalent vanadium ions to the vanadium content in the trivalent vanadium ions is 4.5:5.5 to 5.5:4.5, and that the composition exists within the region circumscribed by a straight line A-B, a straight line B-E, a straight line E-F and a straight line F-A, wherein these lines are formed by joining point A (1.25, 23.2), point B (1.25, 20.4), point E (1.60, 18.4) and point F (1.60, 21.