Cerium oxide (CeO2) nanoparticles have been demonstrated as a potential free-radical scavenger. In the present work, gold (Au) nanoparticles were impregnated by deposition precipitation method on the surface of the combustion synthesized 13-nm CeO2 nanoparticles in order to enhance the free-radical scavenging properties of Au-supported CeO2 nanoparticles (Au/CeO2). Raman spectroscopic calculation for CeO2 and Au/CeO2 showed an oxygen vacancy concentration of 1.22 x 10(21) and 0.80 x 10(21) cm(-3), respectively. The dose- and time-dependent free-radical quenching efficacy of CeO2 and Au/CeO2 nanoparticles was evaluated against hydroxyl, superoxide and nitric oxide using in vitro method. CeO2 and Au/CeO2 nanoparticles exhibited efficient scavenging of hydroxyl and superoxide radicals, but the activity was found to be low against nitric oxide radicals. Both the nanoparticles exhibited a concentration-dependent free-radical scavenging in the range of 0.01-0.0001 mu M and showed a saturation behaviour above 0.1 mu M. Nanoparticle solution aged for 1, 7, 14 and 28 days displayed a lower superoxide and hydroxyl radicals scavenging activity compared to freshly prepared nanoparticle solution while nitric oxide exhibited the opposite behaviour. In comparison, Au/CeO2 showed better radical scavenging activity at lower concentrations than that of CeO2. The observed radical scavenging property can be attributed to the agglomeration as well as changes in surface oxygen vacancy concentration which are important in designing therapeutic agent for oxidative stress complications.