In a former study, we showed that bimetallic Au-Ag nanoparticles supported on TiO2 activated under H-2 at 550 degrees C present the highest activity and a high stability in the reaction of CO oxidation due to a compromise between the particle size and the bimetallic character of the nanoparticles. This paper focuses on the chemical phenomena occurring during the preparation and on the evolution of the Au and Ag species during the activation of the catalysts leading to the formation of bimetallic particles. To characterize the samples, three in situ techniques were used: in situ UV-visible, DRIFTS coupled with CO adsorption and XPS after thermal treatment in adjacent treatment chamber. UV-visible characterization proves that during the preparation of Au-Ag/TiO2 by sequential deposition precipitation (silver first and then gold), silver reduced during the first step suffers a reoxidation due to the chlorides added during the deposition of gold and the formation of AgCl. The UV-visible study during in situ reduction indicates that the bimetallic particles gradually form as the reduction temperatures increases. The evolution of the gold and silver species present at the surface of the metal particles studied by XPS and CO-DRIFTS reveals an increasing proportion of silver atoms in the bimetallic particles during activation under hydrogen while chemical analysis indicates a gradual elimination of the chlorides with temperature. We conclude that the optimum temperature of activation of 550 degrees C for the reaction of CO oxidation is the result of a competition between these positive and negative effects. (C )2015 Elsevier B.V. All rights reserved.