We atom/Pt atom, i.e., coking was not only more facile, but also increased with cluster size. CO TPD used to probe the availability of strong Pt-associated binding sites also showed that Ge addition stabilized the clusters against thermal sintering, and also against CO site blockage by carbon deposited in prior C2D4 TPD runs. Interestingly, the CO TPD experiments also show that for Ptn/alumina, a small amount of carbon deposition also stabilized the clusters against sintering, but unlike Ge, did not stabilize against deactivation by additional carbon deposition. The ISS experiments showed that for Ptn/alumina carbon initially deposits into sites on the clusters, but migrates at high temperatures such that it no longer attenuates He + scattering from Pt. Both the TPD and ISS experiments suggest that much of the cokinginduced loss of strong Pt-associated adsorbate binding sites is the result of electronic, rather than steric effects. Ultimately, therefore, the fact that PtnGem/alumina coking self-limits at ~2 C atoms/cluster, independent of cluster size, is largely a consequence of the combined effects of Ge and C addition.