Aiming at the development of accelerated stability tests of perfluorinated and polyaromatic proton conducting fuel cell polymer membranes, free-radical degradation was initiated by exposure to .OH radicals generated from hydrogen peroxide. All investigations were performed by electron paramagnetic resonance on various monomer and polymer solutions as well as on bare and coated polymers. No signals indicating an .OH attack on Nafion ionomers were detected, in contrast to all polyaromatic membrane materials. Nevertheless, in situ spin trap measurements with Nafion membranes reveal strong signals which formally belong to trapped H atoms but are not related to degradation. Relative to the Faraday electrons from the electrochemical reaction, they appear on a parts per million level.