Dispersed Pt particles supported by amorphous SiO 2 were grown with regular shapes and orientation along the same crystallographic axis (''initial state''). After an oxidizing treatment the samples were heated in 1 bar hydrogen at 873 K. The morphological and structural changes were studied by transmission electron microscopy (TEM). Platinum silicide Pt 3 Si with a L1 2 (Cu 3 Au) structure, monoclinic Pt 3 Si, and tetragonal Pt 1 2 Si 5 were identified after the treatment. A topotactic structural transformation accompanied by the migration of Si from the substrate to the particles is suggested to take place during Pt 3 Si formation. Pt 1 2 Si 5 is formed through melting and recrystallization. The mechanisms of reconstruction of the crystallites are discussed.