Regularly grown Pt particles supported by amorphous SiO2 were heated in hydrogen at 873 K after an oxidising treatment. The morphological and structural changes were studied by electron microscopy. Platinum silicides Pt3Si with L1(2) (Cu3Au) structure, monoclinic Pt3Si and tetragonal Pt12Si5 were identified after the treatment. The mechanisms of coalescence of the particles and the formation of irregular large particles are suggested. A topotactic structural transformation accompanied with the migration of Si from the substrate to the particles are suggested to take place during Pt3Si formation. (C) 2003 Elsevier Science B.V. All rights reserved.