The improvement of redox-flow batteries requires the development of chemically stable and highly conductive separators. Porous glass membranes can be an attractive alternative to the nowadays most common polymeric membranes. Flat porous glass membranes with a pore size in the range from 2 to 50 nm and a thickness of 300 and 500 μm have been used for that purpose. Maximum values for voltage efficiency of 85.1%, coulombic efficiency of 97.9% and energy efficiency of 76.3% at current densities in the range from 20 to 60 mA cm−2 have been achieved. Furthermore, a maximum power density of 95.2 mW cm−2 at a current density of 140 mA cm−2 was gained. These results can be related to small vanadium crossover, high conductivity and chemical stability, confirming the great potential of porous glass membranes for vanadium redox-flow applications.