So-called “Single-Atom” Catalysis represents the ultimate limit of the downsizing of heterogenous catalysts. To be stable on a solid surface, adatoms must form chemical bonds to the support, which modifies their chemical properties from those of the parent metal. In this presentation I will describe our recent research utilizing the surface science approach, in which the model catalysts are studied on a well-defined single crystal Fe3O4(001) support in ultrahigh vacuum conditions [1]. In the first part, I will compare the CO adsorption behavior at various Fe3O4-supported SAC systems (Au1, Ag1, Cu1, Ni1, Pd1, Pt1, Rh1, Ir1), and explain the origin of the differences from metal nanoparticles [2]. Then, I will show that CO exposure leads to chemical sintering of Pt and Pd atoms, but the stabilization of Rh and Ir atoms, with the latter behavior understood by analogy to coordination complexes. Finally, I will show that Rh1(CO)2 gem-dicarbonyl species only form through the breakup of Rh2 dimers via an unstable Rh2(CO)3 intermediate [3]. The results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis. References [1] R. Bliem,…G.S.Parkinson Science 346 (2014), 1215-1218 [2] J.Hulva, M.Meier,…..G.S.Parkinson., Science 371 (2023) 375. [3] C.Wang, P.Sombut, L.Puntscher, Z.Jakub, M.Meier, J.Pavelec, R.Bliem, M.Schmid, U.Diebold, C.Franchini, G.S.Parkinson, Angewandte Chemie International Edition (2024) e202317347.