Measurements by isotopic transient experiments (¹³CO-¹²CO) on the Au/TiO₂ catalyst with an average Au particle size of 3 nm showed that less than 3%of the total surface Au atoms on the Au nanoparticles were responsible for the majority of the catalytic activity. Time-resolved IR spectra collected during the isotopic transient experiments showed that CO adsorbed on Au⁰ species were involved in the reaction pathway, while the surface formate and carbonate species were spectators under these WGS conditions. Based on these results, we conclude that metallic corner Au atoms are the dominant active sites for WGS on Au/TiO₂ catalysts and that CO adsorbed on Au⁰ is an active intermediate. In addition, we present a detailed description of the design, construction, and operation of a transmission FTIR cell capable of performing operando kinetic measurements and isotope switching experiments with a signal decay time of about 95%in 1.5 s.