Geometric structure of Pt and Pt–In catalysts and crystal structures of the active phase.▪ Display omitted •Pt3In and PtIn2 alloys have much high selectivity for ethane dehydrogenation than Pt.•Geometric isolation of Pt sites are suggested to be responsible for high olefin dehydrogenation selectivity.•In promotes the turnover rate of Pt and the promotion depends on the specific alloy structure. The structure of silica supported Pt and Pt–In bimetallic catalysts with nominal In:Pt atomic ratios of 0.7 and 1.4 were determined by in situ synchrotron XAS and XRD. It was seen that the addition of In led to the formation of two different intermetallic alloy phases. At an In:Pt ratio of 0.7 the Pt3In phase with a Cu3Au structure was formed. When the ratio was increased to 1.4 a shell of PtIn2 having a CaF2 structure formed around a core of Pt3In. The catalysts were tested for ethane dehydrogenation at 600°C to determine the effect of alloying on ethylene selectivity and turnover rate (TOR). The monometallic Pt catalysts was 73% selective for ethylene and had an initial TOR of 0.7s−1. Both alloy catalysts were ≈100% selective for dehydrogenation and had higher initial TOR, 2.8s−1 and 1.6s−1 for In:Pt ratio of 0.7 and 1.4, respectively. The increase in selectivity is attributed to the elimination of large Pt ensembles resulting from geometric changes to the catalyst surface upon alloying. Electronic changes due to the formation of Pt–In bonds are thought to be responsible for the increases in TOR in the alloy catalysts.