Nanoceria exhibit redox state-dependent catalase mimetic activity. Cerium oxide nanoparticles (nanoceria) have been shown to protect cells against reactive oxygen species (ROS)-induced damage. The catalase mimetic activity of nanoceria correlates with the level of cerium in the +3 state, in contrast to the relationship between surface charge and superoxide scavenging properties. ROS, including superoxide and hydrogen peroxide, are neutralized by enzymes such as superoxide dismutase (SOD) and catalase. Oxidative stress, caused by an imbalance between ROS and detoxifying enzymes, is linked to various diseases. Nanoceria have been shown to act as SOD mimetics by reacting with superoxide to form hydrogen peroxide. This study demonstrates that nanoceria also exhibit catalase mimetic activity, depending on their redox state. The Amplex Red assay showed that nanoceria preparation B, with higher levels of cerium in the +4 state, exhibited significant catalase mimetic activity, while preparation A, with higher levels of cerium in the +3 state, did not. UV-Vis spectrophotometry confirmed that nanoceria preparation B catalyzed the breakdown of hydrogen peroxide more efficiently. The results suggest that nanoceria with lower +3/+4 redox-state ratios are less efficient in catalase mimetic activity. The study also tested other nanoceria preparations, including nanorods and nanocubes, which also showed catalase mimetic activity. The redox state of nanoceria can be altered by exposure to phosphate solutions, which improved catalase mimetic activity. The findings indicate that nanoceria may protect cells from oxidative stress by acting as catalase mimetics, in addition to exhibiting SOD mimetic activity. Previous studies on nanoceria's antioxidant activity should be reevaluated in terms of their in vivo mechanisms.Nanoceria exhibit redox state-dependent catalase mimetic activity. Cerium oxide nanoparticles (nanoceria) have been shown to protect cells against reactive oxygen species (ROS)-induced damage. The catalase mimetic activity of nanoceria correlates with the level of cerium in the +3 state, in contrast to the relationship between surface charge and superoxide scavenging properties. ROS, including superoxide and hydrogen peroxide, are neutralized by enzymes such as superoxide dismutase (SOD) and catalase. Oxidative stress, caused by an imbalance between ROS and detoxifying enzymes, is linked to various diseases. Nanoceria have been shown to act as SOD mimetics by reacting with superoxide to form hydrogen peroxide. This study demonstrates that nanoceria also exhibit catalase mimetic activity, depending on their redox state. The Amplex Red assay showed that nanoceria preparation B, with higher levels of cerium in the +4 state, exhibited significant catalase mimetic activity, while preparation A, with higher levels of cerium in the +3 state, did not. UV-Vis spectrophotometry confirmed that nanoceria preparation B catalyzed the breakdown of hydrogen peroxide more efficiently. The results suggest that nanoceria with lower +3/+4 redox-state ratios are less efficient in catalase mimetic activity. The study also tested other nanoceria preparations, including nanorods and nanocubes, which also showed catalase mimetic activity. The redox state of nanoceria can be altered by exposure to phosphate solutions, which improved catalase mimetic activity. The findings indicate that nanoceria may protect cells from oxidative stress by acting as catalase mimetics, in addition to exhibiting SOD mimetic activity. Previous studies on nanoceria's antioxidant activity should be reevaluated in terms of their in vivo mechanisms.