It is generally believed that fuel cells will play an important role in energy supply already in the near future. The present work is focused on the polymer electrolyte membrane stability because the membrane is a key component of a fuel cell often limiting the performance of the whole fuel cell system. The aim was to identify the changes which happen on the chemical structure of the membrane after degradation at the cathode and/or anode side of the fuel cell. Previous studies were mainly monitoring the degradation changes by analyzing the water uptake of the fuel cell or by using the liquid ionomers. In this work with the help of solid-state NMR spectroscopy, the direct study of the chemical structure and dynamics of the polymer membranes before and after the degradation test became possible. The novel results from the present work lead to a better understanding of membrane degradation and provide important indications for future developments of membranes with improved performance for alternative energy conversion devices.