The fragmentation of perfluorinated ionomeric membranes during fuel cell (FC) operation is studied in our laboratory by direct electron spin resonance (ESR) and by spin trapping ESR, and interpretation of the results is facilitated by the study of model compounds (MCs). The advantage of this approach is the ability to before the appearance of stable species that can be detected by NMR detect and identify "early events" in the fragmentation process, and other methods. We report a spin trapping ESR study of the 1 fragmentation of Nafion, Aquivion, and 3M membranes in their water dispersions and of the corresponding model compounds in the presence of HO(center dot), using 2-methyl-2-nitrosopropane (MNP) as a spin trap. Hydroxyl radicals were generated by UV irradiation of hydrogen peroxide. In the MCs the presence of both oxygen-centered radicals (OCRs) and carbon-centered radicals (CCRs) adducts as well as di-tert-butyl nitroxide radicals (DTBN, from spin trap decomposition) were detected. The presence of both OCR and CCR adducts is rationalized by the initial generation of OCRs with low stability and their transformation into the more stable CCRs. Addition of beta-cyclodextrin (beta-CD) led to a significant increase of the intensity of the MNP/OCR adducts and in one system also to the complete disappearance of the MNP/CCR adduct, results that we assign to the fast selective encapsulation of OCR adducts in the hydrophobic beta-CD host. In the membrane dispersions the presence of oxygen-centered radical (OCR) adducts and DTBN radicals have been detected; this result is rationalized by the slower rate of transformation of OCR adducts to CCR adducts in the membrane systems.