Résumé : (Graph Presented) The influence of pretreatments with He, O2 and H2 on the physicochemical and catalytic properties of gold supported on hexagonal mesoporous silica (HMS) has been studied by a variety of methods including: XPS, TPR, N2 sorption, UV-visible spectroscopy, FTIR and catalytic testing. The investigations show supported gold to form different states under redox treatments: Au3+ and Au+ ions, neutral and partly charged gold clusters and metal nanoparticles of different sizes. Reducing pretreatments increase the catalytic activity of the samples; while oxidative treatments deactivate the catalyst. Catalytic tests show several regions of the reduced samples with various catalytic behaviours: 20-200°C, 200-400°C and >400°C; this situation is explained by the co-existence of gold active sites of different types. A comparative analysis among spectroscopic and catalytic data allows suggesting Audn δ+ clusters to be responsible for the activity in the low-temperature region (<200°C); neutral Aun clusters are active in the range 200-400°C; and gold nanoparticles may catalyze high-temperature CO oxidation. Au3+ ions are inactive in this process. © 2012 Elsevier Ltd. All rights reserved.