So-called „single-atom“ catalysis (SAC) is currently a hot topic in catalysis research. While the initial aim was to minimise the amount of prescious metals required in reactions, it quickly became clear that the coordination of the single atom affects its electronic structure and thus its reactivity. The heterogeneity of powder-supported catalysts makes it difficult to study structure-function relationships, and this motivates research on model systems where the atomic-scale structure can be precisely defined. In this talk, I will compare how O2 interacts with Pt atoms supported on two model surfaces; Fe3O4(001) [1,2] and Fe2O3(012) [3]. At first glance, the single atoms seem similar. In both cases the Pt are two-fold coordinated to oxygen and slightly oxidized. However, the Pt1/Fe2O3(012) reacts strongly with O2 at room temperature, whereas the Pt1/Fe3O4(001) does not. Using a combination of scanning probe data, XPS, and density functional theory-based calculations, I will show that the location of the 2nd nearest neighbour Fe cations is the deciding factor in whether dissociative adsorption can occur on these model catalysts. These results demonstrate how careful control of the atom/support can have a significant impact on the reactivity of SAC systems. References [1, 2] Science 346, 1215-1218 (2014); Science 371, 375 (2021) [3] ACS energy letters 7, 375-380 (2021) Acknowledgments Work funded by ERC Consolidator Grant “E-SAC” (864628) and FWF SFB TACO (F81).