Catalysts with sub-nano cluster active sites potentially have high mass activity (activity/gram metal) because most or all the cluster atoms are exposed and accessible to reactants. In addition, clusters have physical and chemical properties that can be tuned by varying cluster size, including sizedependent electronic structure and fluxionality (i.e., easy interconversion between isomers) that may enable unique catalysis mechanisms. Using small clusters in catalysts has practical problems, however, including a propensity toward sintering or ripening increasing the cluster size, and the possibility of poisoning by adsorbates that may modify the cluster electronic properties or block reactant binding sites. In particular, carbon deposition (coking) under hydrocarbon-rich conditions is a degradation mechanism that affects many catalysts, and for sub-nano clusters the high temperatures where coking tends to occur also are likely to drive sintering. The focus of this study is understanding the mechanisms by which Ge addition stabilizes sub-nano Pt clusters against both sintering and coking when the clusters are saturated by a model alkene (ethylene), and heated.