SBA-15 functionalization with mercaptopropyltrimethoxysilane has been used to prepare supported gold catalysts for the low temperature CO oxidation reaction. Supports and catalysts have been characterized by chemical analysis, CHS analysis, XRD, TGA, nitrogen adsorption-desorption at 77 K, TEM, CPMAS NMR, XPS and EPR. Catalytic runs have been carried out at atmospheric pressure and 313-623 K and the influence of diverse thermal treatments of the samples prior to reaction has been investigated. The presence of organic residues and the size of the gold nanoparticles strongly affect catalytic activity. Only high-temperature calcination in air followed by treatment under H2 atmosphere leads to active catalysts. After complete elimination of the functionalizing agent, caused by the calcination step, a gold-mediated "activation" process of the silica support takes place during the hydrogen treatment. As a consequence, active catalysts for the low temperature CO oxidation are obtained, even though the size of the Au particles is too large for establishing direct Au-oxygen interactions, usually assumed to be essential for the reaction over silica-supported gold catalysts.