Stimulated by the development of aberration-corrected electron microscopes, single-atom catalysts (SACs) have recently emerged as a promising new class of atom-efficient materials for a variety of reactions usually catalyzed by supported metal nanoparticles [1]. While many demonstrations of successful SAC preparation and evaluation have been published to date, little is known about the stability and dynamics of isolated atoms under reaction conditions. In this work, the stability of -alumina-supported single Pt atoms formed by oxidative treatment of an impregnated Pt precursor has been monitored by operando X-ray absorption spectroscopy (XAS, SOLEIL/ROCK beamline). Their destabilization into subnanometric clusters under reductive treatment has been studied by XAS and environmental scanning transmission electron microscopy (E-STEM, IRCELYON & CLYM). E-STEM experiments under H2 or O2 demonstrate the gas- and temperature-dependent mobility of the Pt entities. DFT calculations (IFPEN Lyon) show that atomic oxygen directly contributes to metal-support adhesion, which is maximized for single Pt atoms, whereas hydrogen adsorbs on platinum only and thereby destabilizes Pt-support bonds, leading to clustering (see figure). To complement the comparison between platinum single atoms and subnanometric clusters, and address the debate on SAC performance [2,3], Pt/γ-Al2O3 SACs were submitted to CO oxidation heating/cooling cycles separated by a reduction treatment. The catalysts were analyzed by operando DRIFTS, operando XAS and STEM. As a result, the initially cationic Pt single atoms are mostly retained during CO oxidation in O2-rich conditions, while they are chemically reduced and partially converted into subnanometric clusters upon treatment with H2 or during CO oxidation in O2-lean conditions. In this contribution, the nature of the active species will be discussed. References: [1] J. Liu, ACS Catal. 7 (2017), p. 34. [2] M. Moses-DeBusk et al., J. Am. Chem. Soc. 135 (2013), p. 12634. [3] K. Ding et al., Science 350 (2015), p. 189.