Surface oxide layers formed on transition metal catalysts are well known as one of the controlling factors in enhancing or suppressing the catalytic activity of metal catalysts. We investigated the growth of a surface oxide layer on two sizes of Rh metal nanoparticles (NPs) as a function of UV-ozone (UV/O-3) dosing as well as how the oxide layer formed on the surface of the Rh NPs affects the catalytic activity for CO oxidation. Monodisperse Rh NPs were synthesized via one-pot polyol reduction using poly(vinylpyrrolidone) as a capping agent. Varying the concentration of the Rh precursors controlled the size of the NPs. The changes that occurred as a function of UV/O-3 dosing were characterized using X-ray photoelectron spectroscopy, which showed that the oxidation state increased with increasing surface modification time. The catalytic activity and activation energy of the two-dimensional Rh NPs arrays were measured as the UV/O-3 exposure time increased. Our reaction studies indicate that the turnover rate of CO oxidation on the Rh NPs is enhanced as the quantity of the surface oxide layer formed during UV/O-3 surface treatment increases, indicating that the oxides grown on the surface of the Rh metal are catalytically active. These results suggest an intriguing way to tune catalytic activity via engineering of the nanoscale surface oxide.