Ceria is a lanthanide series element that can exist as Ce3+ or Ce4+. Cerium oxide nanostructures are being developed for use in industry as catalysts. Nanostructures often behave differently from their corresponding macrostructures and these new behaviors can result in novel and important biological and chemical properties. The free radical theory of aging suggests that free radicals many of which are reactive oxygen species, damage cellular macromolecules. This damage can contribute to degenerative diseases, cancer and aging. Recent research has shown that ceria oxide nanoparticles protected cells from UV and radiation damage as well as decrease intracellular reactive oxygen species in primary cell cultures. The enzyme superoxide dismutase (SOD) protects the cell from free radicals by reacting with superoxide, a reactive oxygen species. Chemicals that catalyze the same reaction as SOD are referred to as SOD mimetics. Another biologically significant free radical is nitric oxide, a reactive nitrogen species. Nitric oxide is an important signaling molecule in both the cardiovascular and nervous system; however it can also cause damage to proteins through nitrosylation. When superoxide and nitric oxide react with each other they form the highly potent free radical, peroxynitrite. This reaction naturally occurs in the phagolysosome of the macrophage and is utilized by the immune system to kill pathogens. Nanoceria was tested for activity against superoxide, nitric oxide and peroxynitrite. The results presented in this work show that ceria oxide nanoparticles exhibit SOD mimetic activity and reduce protein nitrosylation in vitro.