Spin trapping by 5,5-dimethylpyrroline-N-oxide ( DMPO) was used for the detection of radicals in Fenton media in the presence and absence of Nafion perfluorinated ionomers. For ethanol as solvent, the same types of spin adducts were detected in the presence or absence of Nafion. Solvent-derived adducts, DMPO/center dot OC2H5 and DMPO/center dot CH(OH)CH3, were identified, and their presence was rationalized by Fe(III)-catalyzed nucleophilic addition of ethanol to the spin trap and hydrogen abstraction by (OH)-O-center dot radicals; oxygen radical adducts, DMPO/ center dot O-2(-) and DMPO/(OOH)-O-center dot, were also detected. In Fenton media with methanol as solvent ( and no Nafion), the DMPO/O-center dot(2)- adduct dominated immediately after sample preparation, and a mixture consisting of DMPO/center dot-OCH3, DMPO/(CH3)-C-center dot, DMPO/O-center dot(2)-, and DMPO/(OOH)-O-center dot adducts was detected after 30 min. In the presence of Nafion, only the adduct DMPO/(OH)-O-center dot was detected. For water as solvent, only the DMPO/(OH)-O-center dot adduct was detected, in both the absence and the presence of Nafion. The full hyperfine tensor components of this adduct were determined in Fenton media in the presence of Nafion with water and methanol as solvents. In Nafion/ water exposed to the Fenton reagent at 358 K for 3 h, a DMPO adduct of a carbon-centered radical was also identified and assigned to a Nafion-derived fragment; its exact nature is under investigation. Variations of the N-14 and H beta hyperfine splittings of a given adduct with the local polarity were key to the identification of some DMPO adducts, in particular DMPO/O-center dot(2)-. Both (OOH)-O-center dot and O-2(center dot)- adducts, with different N-14 and H beta splittings, were detected simultaneously in some samples, for the first time in the spin trapping literature. Comparison with the results of a direct electron spin resonance study of Nafion exposed to the Fenton reagent indicated that spin trapping by DMPO can provide complementary information on the type of radicals present during Nafion degradation. The spin trapping approach described in this paper is limited, however, to systems that do not contain organic solvents.