Uniform arrays of Au nanoparticles with controlled size and interparticle distance were synthesized by using poiystyrene-b-poly(2-vinylpyridine) as a template and an Ar plasma treatment. These uniform arrays of nanoparticles arc ideal model systems for studying the effects of particle size and interparticle distance on their catalytic activity. Electrooxidation of carbon monoxide (CO) on these particle arrays in CO-saturated 0.1 M NaOH was examined. On particle arrays with a particle size of ca. 4 nm and an interparticle distance varying from 28 to 80 nm, rotating disk electrode (RDE) voltammetric results show that the half-wave potential for CO oxidation shifted to more positive potentials as the interparticle distance increased. This apparent kinetic difference can be explained by the CO diffusion pattern change with the interparticle distance. On particle arrays with a similar interparticle distance but varying size from 2.4 to 9.0 nm, the electrooxidation of CO shows a particle size-dependent activity, with the highest activity obtained on 4.2 nm Au particles, as revealed by the Tafel plot. The Tafel slope also depends on the particle size, with the smallest slope obtained on 4.2 nm particles. The particle size-dependent catalytic activity was tentatively explained in terms of the size-dependent adsorption properties. A brief comparison was made with the results from gas phase CO oxidation on Au nanoparticles.