CO adsorption and further reaction with O-2 on two Au/CeO2 model systems have been investigated by operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) in order to unveil the active sites for CO oxidation. One of the ceria supports was constituted by crystallites depicting a nanocube shape bounded mostly by {100} planes, whilst the second one was consisted of nanocubes whose {110} surfaces were reconstructed into {111}-nanofacets with a zig-zag shape. The deconvolution of the CO adsorption bands and a detailed analysis of the time-evolution of these bands were also carried out. These studies evidenced a fast adsorption of CO over Au degrees and then Aud(delta-)-CO bands appeared during exposure to CO, in parallel with the appearance of Ce3+ features. Such changes suggest that CO adsorption process starts on Au sites on the low coordinated sites of the surface of the Au nanoparticles and their transfer to the interface sites. By comparing the nanostructure of the two catalysts, the Aud(delta-)-CO species could be fruitfully related to CO adsorption on sites located at the Au parallel to CeO2 perimeter. The sample containing gold nanoparticles dispersed over the {111}-nanofaceted CeO2 nanocubes, which retained a higher amount of gold nanoparticles, depicts higher contribution of Aud(delta-)-CO bands than the other Au catalyst in the DRIFT spectra. The dynamic DRIFT study clearly revealed that the CO species adsorbed on Aud(delta-) sites were the most reactive ones in both gold supported systems for CO oxidation. This explains much higher TOF values observed on the catalyst prepared on the surface reconstructed support. A detailed analysis on the reactivity was performed by modulation excitation spectroscopy (MES). The presence of peroxide surface species (O-2(2-)) in nanofaceted {111} could lead to higher catalytic activity than that of gold on the non-reconstructed catalyst.