Pores of poly(vinylidene fluoride) (PVDF) membranes were filled with tetrabutylammonium 4-vinylbenzene sulfonate (TVS), a cross-linking agent, and a radical initiator in dimethyl sulfoxide, followed by radical polymerization and subsequent replacement of tetrabutylammonium ions by protons. This pore-filling method using an organic-soluble sulfonate monomer was very efficient and convenient, as it avoids the highly acidic sulfonation step. The dimensional changes in the length of the PVDF membranes filled with cross-linked sulfonated polystyrene were similar to Nafion 212, but their water uptakes were higher and they exhibited greatly improved mechanical moduli and elongations. Proton conductivities of the modified membranes were also higher than Nafion 212 under identical conditions. (C) 2015 Elsevier B.V. All rights reserved.