To examine the effects of carbon deposition on CO binding, separate sets of Ptn and PtnGem samples For the Ptn samples, comparing the same numbered TPD runs in Figs. and shows the combined effects of thermal sintering/isomerization and carbon deposition during the three C2D4 TPD runs, which would have deposited ~0.7 to 0.8 C atom/Pt atom on average. Compared to CO TPD without prior C2D4 TPD (Fig. ), the most obvious effects were substantial weakening of the desorption between 400 and 600 K, with increased run-to-run stability of the CO desorption. Comparison to runs 4-6 in Fig. , where the samples were heated three times but without carbon deposition, implies that stabilization is an effect of carbon addition to the clusters. Taken together, the data for the Ptn samples in Figs. and show that much of the initial loss of CO binding sites for Ptn/alumina is attributable to thermal sintering/isomerization, with carbon deposition resulting in ~20% additional site loss, but also stabilizing the clusters against further thermal changes. ISS peaks largely result from He + scattering from single atoms in the surface layer, giving welldefined scattered energies that identify the surface atom masses. Sub-surface or multiple scattering events contribute mostly to a featureless background that is weak because He + is less likely to survive such events. Because all samples here had identical numbers of Pt atoms deposited, variations in the Pt