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Atomic Quantum Clusters (AQC) behavior is different from that expected for bulk or nanoparticles (> 2 nm) because their sizes are comparable to the Fermi wavelength of electrons in metals (approx. 2 nm in Au or Ag). Several examples can be found in recent reports about the synthesis of AQC of different metals and their applications, in particular, as heterogeneous catalysts for different reactions.1 One of the main features of metal clusters is that all electrons are covalently bonded and this characteristic allows the appearance of a band-gap giving rise to semiconductor like (or molecular-like) properties, such as photoluminescence. Due to the presence of such band gaps, clusters with a small number of atoms are expected to be very stable. This ?molecular behavior? also justifies their photocatalytic activity, fact that we reported for Ag and Cu-AQC. 2 In addition, Cu5-AQC supported on TiO2 have shown novel catalytic properties for CO2 reduction in the presence of hydrogen.3 In order to fabricate efficient systems that can be used in industrial applications, we need to test how these entities react with the environment, especially when supported AQC are exposed to different atmospheres and temperatures. In particular, with Cu5-AQC/TiO2, several complex interactions are likely to occur, including the interactions between the Cu atoms and the TiO2 surface. This type of interaction gives rise to a doping effect of TiO2 with the Cu5-AQC that produces a change of the bandgap of the semiconductor shifting its absorption to the visible region. In order to study this complex interaction at ?in-situ? conditions, the NAPP end station at the CIRCE beamline is ideal. In this work, we performed NEXAFS Spectroscopy experiments at the Cu L3 edge to study the interaction of Cu atoms from Cu5-AQC/TiO2 with O2 at different temperatures in order to determine the adsorption/desorption properties of these AQC and their stability at different temperatures. We observed that, although XPS 2p spectrum shows the typical features of metallic copper for Cu5-AQC, NEXAFS L3 is sensible to the surface Cu-TiO2 interaction and to the O2 adsorption/desorption process, which is reversible up to 573K. At higher temperature, Cu5-AQC decompose to form CuO. Acknowledgements. ANPCyT (PICT 2015-2285, PICT 2017-1220), UNLP (Proyecto 11/X790) Sincrotrón ALBA, Spain (proposal 2018093158). 1 B. Santiago-González and M. A. López-Quintela, in Functional Nanometer-Sized Clusters of Transition Metals: Synthesis, Properties and Applications, ed. W. Chen and S. Chen, The Royal Society of Chemistry, Cambridge, 2014, pp. 25?50 2 Y. A. Attia, D. Buceta, C. Blanco-Varela, M. B. Mohamed, G. Barone and M. A. López-Quintela, J. Am. Chem. Soc., 2014, 136, 1182?1185; N. Vilar-Vidal, J. Rivas and M. A. López Quintela, Small, 2014, 10, 3632?3636 3 Cong Liu, Bing Yang, Eric Tyo, Soenke Seifert, Janae DeBartolo, Bernd von Issendorff, Peter Zapol, Stefan Vajda, and Larry A. Curtiss J. Am. Chem. Soc., 2015, 137, 8676?8679