 Porous glass (PG) offers the ability to vary pore sizes and modify surfaces, allowing membranes to be tailored for a given electrochemical application. In this contribution, the application of PG in all-vanadium redox-flow batteries (VFB) and the effect of surface modification with sulfonic acid groups were investigated, and the results were compared with those from well-known polymeric membranes. The performance of native and surface-modified PG membranes with pore sizes ranging from 2 to 20 nm and thicknesses of 300 and 500 µm was investigated by examining their self-discharge behavior, polarization curves and area resistance. A maximum power density of 77 mW cm−2 at a current density of 110 mA cm−2 was observed with the modified membrane 505FDS, and this density is approximately half the power density achieved with Nafion™ 117. The results can be related to the small vanadium crossover, high conductivity and chemical stability. Therefore, the great potential of PG membranes as separators in VFBs was shown.