A series of Pt-Sn/gamma-Al2O3 reforming catalysts have been prepared and studied by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). It has been shown that the catalyst performance is dependent on the way in which the catalyst had been prepared. The majority of the tin was in the ionic state as Sn(II) while a minor part of the tin was reduced to the metallic state by platinum. Pt-Sn alloys were found together with platinum particles in the bimetallic catalysts. A significant number of very small TEM-invisible platinum particles less than 1 nm in diameter were present in addition the TEM-visible platinum particles. Particle aggregation and alloy formation occurred during the use of the catalysts in the reforming of octane.