Platinum and tin deposited on gamma-Al2O3, MgO, and Mg(Al)O supports were investigated by CO chemisorption and analytical electron microscopy in the scanning transmission electron microscopy with energy-dispersive X-ray spectrometry (STEM/EDX). Composition and size of individual particles in the 1-nm range are presented and results are compared with dispersion measurements obtained from volumetric chemisorption. We demonstrate that dispersion determined by chemisorption measurements can give unrealistically low values, possibly caused by metal-support interaction, while STEM/EDX reveals the correct size of metal particles. The metal-support interaction seems not to be present on gamma-Al2O3 support but only on Mg(Al)O and MgO supports. The performance of the catalyst in propane dehydrogenation is related to the amount of Pt on the metal particle surface. It is shown that there is a relationship between the composition of metal particles and the activity of the catalyst. The most active is a catalyst that contains metal particles with high Pt content; however, some Sn is necessary for reduced coking and probably for increased stability.