Recent studies suggest that surface oxides on transition metal nanoparticles play an important role in determining the catalytic activity of CO oxidation. In this work, we investigated the influence of surface modification of Rh and Ru nanoparticles on the catalytic activity of CO oxidation using UV-ozone surface treatment. Monodisperse Rh and Ru nanoparticles were synthesized by polyol reduction using poly(vinylpyrrolidone) (PVP) as a capping agent. The size of the nanoparticles was controlled by varying the concentration of the Rh and Ru precursors or using the seeded-growth method. The changes that occurred during UV-ozone surface treatment were characterized with X-ray photoelectron spectroscopy, which showed that the oxidation state increased after surface treatment. The catalytic activity and activation energy of Rh and Ru nanoparticle arrays were measured before and after the chemical modification. Our reaction studies indicate that the turnover rate of CO oxidation on Rh nanoparticles increases by a factor of three after UV-ozone treatment due to the formation of catalytically active Rh oxide. In contrast, the catalytic activity of Ru nanoparticles decreases after UV-ozone treatment, suggesting that the Ru bulk oxide formed during UV-ozone treatment is catalytically inactive. The results suggest an intriguing way to tune catalytic activity via engineering of the nanoscale surface oxide. (C) 2011 Elsevier B.V. All rights reserved.