ESR spectroscopy was used to detect and identify radicals in Nafion perfluorinated membranes exposed to the Fenton reagent based on Ti3+ (TiCl3 + H2O2). This method allowed monitoring the disappearance of the ESR signal from Ti3+ during reaction. The initially formed radical was HOO•, with magnetic parameters g 1 = 2.0261, g 2 = 2.0088, g 3 = 2.0031 (g iso = 2.0127), and a 1 = 5 G at 200 K. The disappearance of this radical above 220 K was accompanied by the appearance of a mixture of peroxy radicals TiOO• and superoxide radicals O2 -. The O2 - radicals were observed only in the dry Nafion after treatment and were stable up to 370 K; the g-tensor components are g 1 = 2.0190, g 2 = 2.0092, and g 3 = 2.0036 (g iso = 2.0106) at 300 K. In samples swollen by the hydrogen peroxide solution, signals from the TiOO• radicals predominated, with g-tensor components g 1 = 2.0228, g 2 = 2.0092, and g 3 = 2.0046 (g iso = 2.0122) at 200 K. A dynamical process lead to averaging of the g anisotropy of TiOO• radicals in the temperature range 220−320 K. The dynamics mechanism was identified with the “cubic jump” model:  rotation of the O−O fragment about an axis equally inclined to the principal directions of the g tensor. A broad signal (line width ≈ 84 G, g = 2.0023) appeared in slightly dried samples after ≈14 days and increased in intensity after 92 days; this signal is thought to represent fluorinated alkyl radicals, formed by attack of oxygen radicals on the polymer chain. In the Fenton reagent (in the absence of Nafion) radicals are detected only for high concentrations of hydrogen peroxide, 30% w/v. In addition, the HOO• radicals formed are less stable, and the peroxy radicals TiOO• are the only species detected at and above 200 K. The presence of the Nafion membranes has important effects on both the type of radicals formed and their thermal stability.