Dispersed Pt particles supported by amorphous SiO2 were grown with regular shapes and orientation along the same crystallographic axis ("initial state"). After an oxidizing treatment the samples were heated in I bar hydrogen at 873 K. The morphological and structural changes were studied by transmission electron microscopy (TEM). Platinum silicide Pt3Si with a L1(2) (Cu3Au) structure, monoclinic Pt3Si, and tetragonal Pt12Si5 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 Pt3Si formation. Pt12Si5 is formed through melting and recrystallization. The mechanisms of reconstruction of the crystallites are discussed. (C) 2003 Elsevier Inc. All rights reserved.