The Pt/g-alumina system is representative of many common heterogeneous catalysts, which consist of transition metals dispersed on a high surface-area support. For example, catalytic reduction and oxidation of automotive pollutants is accomplished with platinum (Pt) or rhodium (Rh) dispersed on a galumina surface. Olefin cracking is carried out on catalysts formed by dispersal of transition-metal atoms on insulating supports, such as zeolites or alumina. The Cr/alumina system is used for dehydrogenation of alkanes to form starting materials for chemical synthesis. Prerequisite to the nonempirical design and refinement of improved catalysts is the identification of the atomic-scale structure and properties of the catalytically active sites. This has become a major industrial research priority. It has long been recognized that electron microscopy is a powerful tool for investigating supported catalysts. 7] Z-contrast imaging in scanning transmission electron microscopy (ZSTEM, where Z denotes nuclear charge) is capable of imaging individual atoms on real (i.e. , not idealized) catalytic supports. Application of Z-STEM to the Pt/g-alumina catalytic system suggested the presence of intriguing atomic-scale structures. In addition to isolated Pt atoms, trimers of Pt are observed with preferred shapes that are inconsistent with those of gasphase transition metal clusters and exhibit atypical Pt–Pt distances. Trimers of Pt on g-alumina have also been inferred from average coordination number as determined by extended X-ray absorption fine structure measurements. These supported clusters are qualitatively different from larger Pt aggregates on g-alumina revealed by transmission electron microscopy (TEM) studies. First-principles calculations have been used to yield detailed structural information about supported metal clusters. 13] For example, theoretical investigations have been reported for small clusters of Pt and Pd on a-alumina. Herein, we report new subangstrom-resolution Z-STEM images of the Pt/g-alumina system that confirm the presence of the perplexing Pt3 structures. We then apply first-principles density functional theory calculations to test several hypotheses about the structure of these supported clusters. These studies reveal three key features of the Pt/g-alumina system: 1) Oxygen termination of the alumina surface is favored; 2) Adatom Pt structures are thermodynamically preferred over substitutional Pt structures; 3) Pt3 structures with anomalously long Pt–Pt distances are stabilized by OH caps.