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 A mu m 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 (TM) 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. [GRAPHICS] .