The catalytic activity for the oxidation of CO of Ag, and Au-Ag catalysts supported on pure synthetic rutile, and on a titania containing anatase and brookite is reported. The catalysts were characterized with X-ray powder diffraction and refinement of the crystalline structures by using the Rietveld technique. The catalytic activity of silver catalysts increased as the crystallite size decreased; however, they were much less active than gold catalysts with similar metal concentrations. When the silver was supported on rutile the active phase was Ag2O, whereas in the titania containing anatase and brookite the active phase could be probably metallic silver. The Au-Ag catalysts were able to catalyze the CO oxidation at low temperatures with long-term stability and durability. The interaction between gold and silver atoms in the metallic particles explains the catalytic performance of these catalysts. The most active catalysts were those prepared by sequential deposition (first silver and then gold) and activated in hydrogen. The refinement of the crystalline phases suggests that gold and silver atoms formed alloys that interacted strongly with the support and were stable in air at room temperature. As in the case of the catalysts with only gold, the catalytic activity of the bimetallic Au-Ag catalysts increased as the dimensions of their crystallites decreased. (C) 2012 Elsevier B.V. All rights reserved.