Ultrafine ceria nanoparticles in the size regime of 3-5 nm were synthesized by chemical methods. The oxygen free radical scavenging properties of ceria nanoparticle was studied using electron paramagnetic resonance technique at the concentrations of 1 mM and 10 mu M in the presence of a spin trap. The hydroxyl radical scavenging properties of ceria nanoparticles were found to vary, depending on the concentrations. At lower concentration the catalytic effect was found to be better compared to that at higher concentration, presumably due to the agglomeration at higher concentrations. (c) 2007 Published by Elsevier B.V.