We previously examined deactivation of sub-nano Ptn/alumina (n = 4,7,8) and Ptn/silica (n = 4,7) model catalysts by a combination of sintering and coking, during repeated ethylene temperatureprogrammed desorption (TPD) experiments. Density functional theory (DFT) with PBE functional and PAW pseudopotentials was used to examine ethylene binding and its effects on cluster structure, stability, and fluxionality. The clusters were found to deactivate rapidly, as shown by the loss of strong ethylene chemisorption sites, with effects from both carbon deposition and sintering. In addition, sintering was found to be size-dependent for Ptn/alumina, with Pt4 and Pt7 showing enhanced (but still imperfect) stability at temperatures to 750 K, compared to neighboring cluster sizes. In those experiments, sintering was inferred from changes in both numbers of exposed Pt binding sites estimated by TPD, and in the fraction of Pt atoms in the surface layer, estimated by He + ion scattering (ISS). Imaging would provide a more direct way of observing sintering but supports such as the alumina/Ta(110) used here are too thick for transmission electron microscopy (TEM). We have reported scanning TEM (STEM) imaging of Ptn deposited in UHV on carbon and alumina thin film TEM grids. The STEM was not aberration corrected, but the cluster spot sizes, densities, and stability in repeated scans suggested that the air-exposed Pt clusters were at least reasonably stable under the ebeam at room temperature, but we were not able to test the effects of heating. The relevance of these measurements to sintering on a different support under the effects of heating and/or adsorbate binding and desorption is unclear, thus the present work also relies on TPD and ISS to provide in situ probes of sintering.