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Control of metal–support interactions is one of the major challenges in heterogeneous catalysis. In this study, we explored the effect of Eu doping on the metal–support interactions in Au/Ce1–x Eu x O2 catalysts applied in the oxidation of CO. We probe the catalysts with CO under realistic conditions by near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). NAP-XPS allows in situ investigation of the surface reducibility of the ceria support and charging of the Au nanoparticles. Both parameters are crucial, as they are responsible for the efficiency of Au/ceria catalysts in the CO oxidation. We found that doping of the ceria support with Eu decreases the reducibility of Ce4+ in the Au/Ce1–x Eu x O2 system and the population of Auδ+ sites. We identified the available CO adsorption sites on Au/ceria (Auδ−, Au0, and Auδ+) in the typical temperature window of the CO oxidation reaction by in situ DRIFTS. We found that the difference in the catalytic activity of Au/CeO2 and Au/Ce0.80Eu0.20O2 does not origin from an electronic effect of the Eu dopant or different adsorption sites but from the size of the Au nanoparticles. The most active Au nanoparticles are very small, reactive Au nanoparticles, which are exclusively present on the undoped ceria support.