Kinetic, microcalorimetric, and Mossbauer spectroscopic studies were conducted to investigate the factors controlling the selectivity of isobutane conversion over silica-supported Pt and Pt/Sn catalysts. Kinetic studies show that addition of Sn to Pt enhances the selectivity for isobutylene formation at temperatures near 700 K. Mossbauer spectroscopic studies show that Sn interacts with Pt to produce a Pt/Sn alloy. Microcalorimetric investigations show that addition of Sn reduces the number of sites that strongly interact with hydrogen or carbon monoxide. The elimination of stronger adsorption sites may be caused by preferential blocking of sites by tin or weakening of sites due to ligand effects. However, strong adsorption sites are present on a Pt/Sn catalyst that exhibits high dehydrogenation selectivity. Microcalorimetric studies of ethylene adsorption suggest that addition of Sn inhibits the formation of highly dehydrogenated surface species. These results suggest that high dehydrogenation selectivity achieved by addition of Sn to Pt is caused by a decrease in the size of surface Pt ensembles.