The catalytic dehydrogenation of propane has been studied over Pt and Pt-Sn catalysts supported on gamma-Al2O3 and SiO2. Without the promoter Pt shows the same initial specific activity (TOF) on both supports, but deactivates rapidly due to coking. The effect of Sn as a promoter depends on the support. On gamma-Al2O3 tin interacts with the support and is stabilized in an oxidation state > 0. The result is an increase in Pt dispersion and reduced deactivation without any change in the initial specific activity in dehydrogenation. The selectivity to propene is strongly enhanced by tin, particularly due to blocking or poisoning of acid sites on the support. On SiO2 the Sn is more readily reduced, and alloy formation is possible. This leads to a similar increase in Pt dispersion and improved catalytic stability, but also to a strong reduction in the specific activity. The change in catalytic stability on both supports is parallelled by a dramatic change in the hydrogen adsorption properties, as seen from the TPD profiles after reduction.