The physico-chemical properties and activity of Ce-Zr mixed oxides, CeO2 and ZrO2 in CO oxidation have been studied considering both their usefulness as supports for Au nanoparticles and their contribution to the reaction. A series of Ce1-xZrxO2 (x = 0. 0.25, 0.5. 0.75, 1) oxides has been prepared by sol-gel like method and tested in CO oxidation. Highly uniform, nanosized, Ce-Zr solid solutions were obtained. The activity of mixed oxides in CO oxidation was found to be dependent on Ce/Zr molar ratio and related to their reducibility and/or oxygen mobility. CeO2 and Ce0.75Zr0.25O2, characterized by the cubic crystalline phase show the highest activity in CO oxidation. It suggests that the presence of a cubic crystalline phase in Ce-Zr solid solution improves its catalytic activity in CO oxidation. The relation between the physico-chemical properties of the supports and the catalytic performance of Au/Ce1-xZrxO2 catalysts in CO oxidation reaction has been investigated. Gold was deposited by the direct anionic exchange (DAE) method. The role of the support in the creation of catalytic performance of supported Au nanoparticles in CO oxidation was significant. A direct correlation between activity and catalysts reducibility was observed. Ceria, which is susceptible to the reduction at the lowest temperature, in the presence of highly dispersed Au nanoparticles, appears to be responsible for the activity of the studied catalysts. CeO2-ZrO2 mixed oxides are promising supports for Au nanoparticlcs in CO oxidation whose activity is found to be dependent on Ce/Zr molar ratio. (c) 2008 Elsevier B.V. All rights reserved.