Many diseases are associated with oxidative stress, which occurs when the production of reactive oxygen species (ROS) overwhelms the scavenging ability of an organism. Here, we evaluated the carbon nanoparticle antioxidant properties of poly(ethylene glycolated) hydrophilic carbon clusters (PEG-HCCs) by electron paramagnetic resonance (EPR) spectroscopy, oxygen electrode, and spectrophotometric assays. These carbon nanoparticles have 1 equivalent of stable radical and showed superoxide (O₂^•−) dismutase-like properties yet were inert to nitric oxide (NO^•) as well as peroxynitrite (ONOO⁻). Thus, PEG-HCCs can act as selective antioxidants that do not require regeneration by enzymes. Our steady-state kinetic assay using KO₂ and direct freeze-trap EPR to follow its decay removed the rate-limiting substrate provision, thus enabling determination of the remarkable intrinsic turnover numbers of O₂^•− to O₂ by PEG-HCCs at >20,000 s⁻¹. The major products of this catalytic turnover are O₂ and H₂O₂, making the PEG-HCCs a biomimetic superoxide dismutase.