Pt and Pt-Sn catalysts supported on SiO2 and gamma-Al2O3 and prepared by impregnation have been studied by temperature-programmed reduction (TPR), hydrogen chemisorption, temperature-programmed desorption (TPD), and catalytic dehydrogenation of propane. Without the promoter Pt shows the same initial specific activity 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 an improved stability of the catalytic activity without any change in the initial specific activity in dehydrogenation. 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 paralleled by a dramatic change in the hydrogen adsorption properties, as seen from the TPD profiles after reduction.