Perfluorosulfonic acid (PFSA) membranes subjected to aging in air form a degradation product with decreased water content and water permeance. FTIR kinetics studies performed on a range of membrane thicknesses shows that reaction exhibits zero-order kinetics, has a very weak temperature dependence and a strong dependence on air velocity. Reactions performed in nitrogen or synthetic air do not form the degradation product. The kinetic data, in combination FTIR spectra of deuterium exchanged products indicate that the degradation product results from an acid-base reaction between ultra-trace levels of atmospheric ammonia and the sulfonic acid groups of the PFSA membrane. (C) The Author(s) 2018. Published by ECS.