Nonstoichiometric CeO2 and Ce0.25Zr0.75O2 nanoparticles with varying surface concentrations of Ce3+ were synthesized. Their surface Ce3+ concentration was measured by XPS, and their surface oxygen vacancy concentrations and grain size were estimated using Raman spectroscopy. The surface oxygen vacancy concentration was found to correlate well with grain size and surface Ce3+ concentration. When incorporated into a Nafion polymer electrolyte membrane (PEM), the added nonstoichiometric ceria nanoparticles effectively scavenged PEM-degradation-inducing free radical reactive oxygen species (ROS) formed during fuel cell operation. A 3-fold increase in the surface oxygen vacancy concentration resulted in an order of magnitude enhancement in the efficacy of free radical ROS scavenging by the nanoparticles. Overall, the macroscopic PEM degradation mitigation rate was lowered by up to 2 orders of magnitude using nonstoichiometric ceria nanoparticles with high surface oxygen vacancy concentrations