The stability of Nation, stabilized Nation (StNafion), and 3M and Aquivion membranes to attack by the hydroxyl radical, HO center dot was compared in their aqueous dispersions at 300 K. HO radicals were generated by UV irradiation of hydrogen peroxide (H2O2), and radicals were detected by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Two types of adducts were detected in all dispersions: DMPO/OH and DMPO/CCR, an adduct of carbon-centered radicals (CCRs) derived from the polymers. The competitive kinetics (CK) approach that has been adapted for ranking the polymer stability leads to the determination of their reaction rate constant with hydroxyl radicals. The experiments consisted of measuring the concentration of the DMPO/OH adduct in the presence of the polymers, which are considered to be "competitors" that react with HO center dot radicals and compete with the spin trapping reaction HO center dot + DMPO -> DMPO/OH. The results indicated an improved stability in StNafion compared with Nation. The largest effect was, however, detected for the 3M and Aquivion polymers, which were significantly more stable. Examination of the hyperfine splittings of DMPO/CCR and of published reports suggests that the absence of the ether group and of the tertiary carbon in the side chain appears to be responsible for the greater stability of these ionomers. The addition of Ce(III) to the polymer dispersions led to a lower total concentration of the adducts DMPO/OH and DMPO/CCR. This result provided evidence for scavenging of HO center dot radicals by Ce(III) that also leads to a lower concentration of the DMPO/CCR adduct. The CK approach was used to determine the reaction rate constant of Ce(III) with the HO center dot radicals: k(Ce) = 2.8 x 10(8) M-1 s(-1), in good agreement with the literature value of 3 x 10(8) M-1 s(-1). The present experiments reaffirmed the centrality of HO' radicals as an aggressive oxygen species involved in membrane degradation, the important effect of the side chain structure on membrane stability, and the stabilizing presence of Ce(III).