The size of the gold particles is a very important parameter to get active catalysts. Only the methods able to produce small particles (< 5 nm) supported on oxides, with a minimum of chlorides, lead to performing catalysts, especially for CO oxidation. In addition to the preparation methods, the gold particle size depends on other parameters, for example, the conditions of thermal treatment. This work focused on the parameters of thermal treatment that influence the gold particle size in Au/TiO2 samples prepared by different methods. It was found that the gold particle size decreases when the gas flow increases, when the amount of sample decreases, or when H-2 or At is used instead of air. The particle size slightly increases with the treatment temperature. The two first parameters cause more drastic changes in particle size than the treatment temperature. Once gold metal particles are obtained, their size can evolve in the presence of light or in ambient air. It is proposed to store them in a desiccator, under vacuum and in the dark. Possible evolutions of the samples during characterization (exposure to electrons or photons beams) are also discussed. (c) 2005 Elsevier B.V. All rights reserved.