The Au/HY (4 wt.%) system, prepared by autoreduction of [Au(en)₂]³⁺ in inert gas flow at 423 K, has been studied by TEM, ¹²⁹Xe NMR and diffuse reflectance infrared spectroscopy of adsorbed CO. 80% of the metallic clusters were in the 1–4 nm size range, and the average linear diameter was found to be 2.6 nm. Some large particles (>5 nm) were also seen. Xenon adsorption and ¹²⁹Xe NMR spectroscopy proved the presence of gold clusters inside HY cavities and showed clearly that gold clusters are weak adsorption sites for xenon. Heterogeneity in the particle size distribution was suggested by ¹²⁹Xe NMR spectroscopy. Diffuse reflectance IR spectroscopy showed an intense asymmetric absorption band of CO linearly adsorbed on the gold surface. Two weak shoulders at ca. 2110–2120 and 2170 cm⁻¹ were ascribed to neutral particles and oxidized gold sites, respectively. The main absorption band at about 2140 cm⁻¹ is observed for the first time for CO adsorbed on a gold surface and was attributed to CO species interacting with electron-deficient gold particles Au_n^σ+ inside the HY lattice and in contact with protons. CO was more strongly adsorbed on these Au_n^σ+ particles than on other gold sites; the corresponding IR stretching vibration was observed even after evacuation for 30 min at room temperature.