In the archetypal hydrogen–oxygen proton exchange membrane fuel cell design, a proton-conducting polymer membrane (typically Nafion) contains the electrolyte solution that separates the anode and cathode sides. How water is distributed in the Nafion membrane is a continuing subject of investigation by a range of computational, spectroscopic and imaging techniques. INS spectroscopy will largely only “see” the modes associated with the acidic proton in the dry ionomer and the water and/or hydroxonium ions in the wet material. The aim of this proposal is to characterise the modes associated with the acid proton in Nafion and how the water is present in the polymer as a function of concentration. We also wish to measure trifluoromethanesulfonic acid and perfluoro(3-oxapentane-1-sulfonic acid, which are model compounds of the sulfonic acid functionality and the pendant side chain.