A dynamic Monte Carlo simulation including real time dependence, thermal effects and different structural features of nano-sized crystallites has been developed. The simulations results are compared with experimental data of CO oxidation over Pt/SiO2 catalysts. A model including the various type of sites found in small crystallites of supported catalysts is used to analyze the structure sensitivity and the rapid transition from low to high activity during CO oxidation. A good agreement between the simulation and experimental results, provide an explanation of the crystallite size and thermal effects experimentally observed. (C) 2004 Elsevier Ltd. All rights reserved.