Sulfonated polybenzimidazole (s-PBI) gel membranes were prepared and shown to have a high stability in concentrated sulfuric acid and strongly oxidizing vanadium (V) solutions. These membranes were considered candidates for use in vanadium redox flow batteries, and compared to the commonly used "conventionally imbibed" meta-polybenzimidazole (m-PBI) membranes cast from N,N'-dimethylacetamide (DMAc) solutions. The s-PBI membranes exhibited high conductivities (up to similar to 240 mS cm(-1)) and low performance degradation during in-cell testing. (C) The Author(s) 2019. Published by ECS.