A novel setup based on electron paramagnetic resonance is used for the investigation of the hydroxyl radical driven degradation on contact of gaseous hydrogen peroxide with sulfonated polyetherketones. This method is more sensitive than mass loss investigations and allows obtaining results 300 times faster and with higher accuracy. Furthermore, it reveals detailed information about the character and amount of paramagnetic intermediates formed during degradation. The spectra are composed of three signals which are attributed to sunny!, phenyl, and phenoxy type radicals. Drying the membranes in the absence of hydrogen peroxide leads to internal tension of the ionomer material and to bond breakage, and therefore also to a small amount of radical defects. The presence of the peroxide at first leads to a linear increase in radical concentration. The kinetics is compatible with a dissociative Langmuir type chemisorption process. With progressing degradation time, a steady state is reached due to saturation of the surface and to radical termination. It is accompanied by cross-linking the polymer and increasing its brittleness. The surface layer then breaks off and leaves the sample in pieces (peel-off effect), providing a significant mass loss at random times. Between the two studied commercial membranes, sulfonated poly(phthalazinone ether ketone) was found to be more inert toward hydroxyl radicals than sullonated poly(ether ether ketone).