This study investigates the catalase mimetic activity of cerium oxide nanoparticles (nanoceria) and finds that this activity is redox-state dependent. Specifically, nanoceria exhibit catalase mimetic activity when they have a higher level of cerium in the +3 oxidation state, which is associated with surface oxygen vacancies. The catalytic efficiency of nanoceria in scavenging superoxide anions and hydrogen peroxide is influenced by the redox state of cerium. Higher levels of cerium in the +3 state result in more efficient catalase mimetic activity, while lower levels of +3 state cerium are less effective. The study also demonstrates that altering the redox state of nanoceria by exposure to phosphate solutions can improve their catalase mimetic activity. These findings suggest that the ability of nanoceria to act as catalase mimetics may be a key mechanism by which they protect cells from oxidative stress.This study investigates the catalase mimetic activity of cerium oxide nanoparticles (nanoceria) and finds that this activity is redox-state dependent. Specifically, nanoceria exhibit catalase mimetic activity when they have a higher level of cerium in the +3 oxidation state, which is associated with surface oxygen vacancies. The catalytic efficiency of nanoceria in scavenging superoxide anions and hydrogen peroxide is influenced by the redox state of cerium. Higher levels of cerium in the +3 state result in more efficient catalase mimetic activity, while lower levels of +3 state cerium are less effective. The study also demonstrates that altering the redox state of nanoceria by exposure to phosphate solutions can improve their catalase mimetic activity. These findings suggest that the ability of nanoceria to act as catalase mimetics may be a key mechanism by which they protect cells from oxidative stress.