The size effect of ZnO nanoparticles on catalytic activity of ZnO/MgO solids was studied in CO oxidation. We have developed a simple method for preparing supported materials containing ZnO nanoparticles of a controllable size with the same chemical composition, structure, and shape. We revealed that the turnover frequency of CO oxidation on ZnO nanoparticles of variable average radius (2.0-2.3 nm) shows bell-shaped dependence on ZnO nanoparticle size. A dependence of catalytic activity of ZnO nanoparticles on their electronic structure is found. This allows us to explain a maximum in the CO oxidation activity observed for ZnO nanoparticles with the size of about 2.2 nm within the framework of the quantum confinement effect. (C) 2010 Elsevier B.V. All rights reserved.