The study investigates the role of cerium redox state in the superoxide dismutase (SOD) mimetic activity of nanoceria. Electron paramagnetic resonance (EPR) analysis confirms that nanoceria has SOD mimetic activity, specifically in scavenging superoxide radicals. X-ray photoelectron spectroscopy (XPS) and UV-visible spectroscopy show that a decrease in the Ce 3+/4+ ratio, which correlates with a loss of SOD mimetic activity, is directly related to the surface oxidation state of nanoceria. The results suggest that the ability of nanoceria to scavenge superoxide is directly related to the concentration of cerium (III) at the particle surface. The study also demonstrates that hydrogen peroxide treatment temporarily inactivates the SOD mimetic activity of nanoceria, but this inactivation is transient and the activity can be regained over time. The findings provide insights into the mechanism behind the SOD mimetic activity of nanoceria and highlight the importance of the surface-associated cerium (III) sites in this activity.The study investigates the role of cerium redox state in the superoxide dismutase (SOD) mimetic activity of nanoceria. Electron paramagnetic resonance (EPR) analysis confirms that nanoceria has SOD mimetic activity, specifically in scavenging superoxide radicals. X-ray photoelectron spectroscopy (XPS) and UV-visible spectroscopy show that a decrease in the Ce 3+/4+ ratio, which correlates with a loss of SOD mimetic activity, is directly related to the surface oxidation state of nanoceria. The results suggest that the ability of nanoceria to scavenge superoxide is directly related to the concentration of cerium (III) at the particle surface. The study also demonstrates that hydrogen peroxide treatment temporarily inactivates the SOD mimetic activity of nanoceria, but this inactivation is transient and the activity can be regained over time. The findings provide insights into the mechanism behind the SOD mimetic activity of nanoceria and highlight the importance of the surface-associated cerium (III) sites in this activity.