In CO oxidation, Cu/CuOx species are known to increase the catalytic activity and improve the sinteringresistance of Au supported catalysts, particularly in the case of non-reducible oxide supports such as SiO2. Nonetheless, there is a lack of detailed information about the impact of Cu species on the stability of Au nanoparticles (NPs) supported on CeO2, one of the most important reducible oxide supports used in catalysis. Due to the reducibility of this oxide, the stability of the metallic phase and the metal-support interaction are strongly dependent on the atmosphere, i.e., oxidizing or reducing conditions. Besides, how residual contaminants from the preparation of the catalysts, such as chlorine, may affect the catalytic activity is not clear for the AuCu/CeO2 system. In this work, we used Au and AuCu colloidal NPs with initially well-defined size and shape to produce model CeO2 supported catalysts to better comprehend the role of the Cu and chlorine on the sintering-resistance of Au NPs and its impact in the catalytic performance of CO oxidation (CO-OX). We show that while the activation of the catalysts by hydrogen enhanced the sintering-resistance of the metallic phase in AuCu/CeO2, activation by oxygen was more effective to generate the most active sites. The presence of chlorine induced the sintering of the metallic phase, particularly after the activation by hydrogen. Besides, the results suggest that the chlorine partially poisoned the Au-CuOx-CeO2 interfacial species, decreasing the catalytic performance.