The vanadium ion permeability of various ion-conducting polymer membranes was determined using a home-made apparatus. The study includes proton-conducting sulfonated poly(ether ether ketone) (SPEEK) with various cross-linking degrees, anion-conducting membranes, such as polysulfone with quaternary ammonium groups (PSU-QA) and sulfaminated PEEK (SA-PEEK), and amphoteric membranes based on PEEK containing both sulfonic and sulfonamide groups (SAM-PEEK) that can conduct both cations and anions. The polymer structure was investigated by NMR spectroscopy. The permeability of SPEEK decreases with the cross-linking degree and can be up to two orders below the permeability of a Nafion 117 reference membrane (1.4 . 10(-6) cm(2) min(-1)). The cation permeability of SA-PEEK attains values as low as 5.0 . 10(-10) cm(2) min(-1), whereas amphoteric SAM-PEEK has a cation permeability of 7.0 . 10(-10) cm(2) min(-1). A factor or merit is introduced, defined as the ratio of ion conductivity and ion permeability; the highest Value, corresponding to the best compromise of high ion conductivity and low permeability of electrochemically active ions, is found for highly cross-linked SPEEK, SA-PEEK and SAM-PEEK membranes. The influence of the polymer backbone (PEEK or PSU), the degree of cross-linking, determined by NMR spectroscopy, and the grafted ionic groups (sulfonic acid, sulfammonium, and quaternary ammonium) on the cation permeability is disaissed. (C) 2016 Elsevier B.V. All rights reserved.