Abstract: Novel catalytic properties are expected from atomic quantum clusters (AQC), which could lead to the development of new catalysts at the sub-nanometer scale. To evaluate their potential and real applications, studies about their stability and reactivity, associated with their support and thermodynamic conditions, become mandatory, especially through advanced in situ characterizations methodologies. The same requirements are needed to elucidate possible catalytic mechanisms. For that, we present a study employing NAP-XPS and XAS synchrotron-based techniques, on supported Cu5 and Ag5 AQCs, their stability and ability to sulfur oxidation reactions, and potential applications in the fields of nanomedicine and industrial catalysis.