The degradation of two different types of perfluorinated polymer membranes, Nafion and Hyflon Ion, has been examined by solid-state F-19 and C-13 NMR spectroscopy. This spectroscopic technique is demonstrated to be a valuable tool for the study of the membrane structure and its alterations after in situ degradation in a fuel cell. The structural changes in different parts of the polymers are clearly distinguished, which provides unique insight into details of the degradation processes. The experimental NMR spectra prove that degradation mostly takes place within the polymer side chains, as reflected by the intensity losses of NMR signals associated with SO3H, CF3, OCF2 and CF groups. The integral degree of degradation is found to decrease with increasing membrane thickness while for a given thickness, Hyflon Ion appears to degrade less than Nafion. (c) 2008 Elsevier B.V. All rights reserved.