We present experiments in a fuel cell (FC) inserted in the resonator of the electron spin resonance (ESR) spectrometer, which allowed separate monitoring of radical formation at anode and cathode sides. The in situ FC was operated at 300 K under closed- and open-circuit voltage conditions, CCV and OCV, respectively, with a membrane-electrode assembly (MEA) based on sulfonated poly(ether ether ketone) (SPEEK). The formation of radicals was monitored by spin trapping ESR, with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trap. The magnetic parameters and the relative intensities of DMPO adducts were determined by simulation of the ESR spectra. The major adducts detected at both electrodes were DMPO/OOH and DMPO/H. The generation of the DMPO/OOH adduct at the anode can be explained by reaction of crossover oxygen with hydrogen atoms formed at the Pt catalyst; at the cathode this adduct can be generated by H-2 crossover to the cathode, reaction at the Pt catalyst, and reaction with O-2. The generation of the DMPO/OOH adduct depends therefore on gas crossover: at the anode from crossover O-2 and at the cathode from crossover H-2. Results indicated that the crossover rates are enhanced in the more highly hydrated membrane. No membrane-derived carbon-centered or oxygen-centered radical adducts were detected in the in situ FC. However, in SPEEK membranes directly exposed to hydroxyl radicals in the presence of DMPO, the DMPO/Ph and DMPO/OPh adducts were detected, indicating poor chemical stability of SPEEK in the ex situ Fenton test. The conclusion is that hydrocarbon membranes such as SPEEK demonstrate different chemical stabilities in the two tests.