We present experiments in all in situ fuel cell (FC) inserted in the resonator of the ESR spectrometer that 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 Nafion 117 10% neutralized by Ce(III) and Pt as catalyst, notation MEA/Ce. The presence of the unstable intermediates was determined by addition of 5.5-dimethylpyrroline-N-oxide (DMPO) as a spin trap, simulation of the spectra from the DMPO adducts, and analysis of the corresponding magnetic parameters and relative intensities. The main objective of these experiments was to identify early events in the mitigation mechanism of Ce(III) on Nafion degradation. The present results were therefore compared to an in situ FC based on Nafion 117 in the acid form, notation MEA/H (Danilezuk et al. J. Phys. Chem. B 2009, 113, 8031-8042) that served as baseline. The differences between the two types of experiments are significant. (a) Spin trapping indicated the presence of different radicals: HO center dot for MEA/H and HOO center dot for MEA/Ce. (b) DMPO/CCR, the adduct of a carbon-centered radical (CCR) generated by membrane fragmentation, is absent for the MEA/Ce FC, but was detected in the MEA/H FC under similar operating conditions. (c) The intensity of all adducts is much lower in the MEA/Ce FC. The absence of DMPO/OH for MEA/Ce was explained by scavenging of the aggressive HO center dot radicals by Ce(III); this process also generates Ce(IV). The presence of DMPO/OOH for MEA/Ce was rationalized by Ce(IV) oxidation of H2O2 leading to the formation of HOO center dot. The absence of DMPO/CCR is a result of HO center dot scavenging and the formation of the less aggressive oxidant, HOO center dot in MEA/Ce. The low intensity of all adducts is also a result of HO center dot scavenging by Ce(III). Ce(III) is an effective stabilizer because of the Ce(III)/Ce(IV) couple redox chemistry, which allows significant stabilization with a low stabilizer concentration.