Aqueous solutions with Au3+ and Pt4+ ions and additives of surfactants (SDS or PEG-MS) were irradiated with an ultrasound 2 at 200 kHz with an input power of 4.2 W/cm(2) and colloidal nanoparticles were prepared. The prepared nanoparticles were characterized by XRD, TEM, HRTEM, EDX and Au-197 Mossbauer spectroscopy. It was found that the structures of nanoparticles were changed with the surfactants; Au and Pt nanoparticles were prepared individually by using SDS, and bimetallic Au/Pt alloy nanoparticles with a core-shell structure were produced in the presence of PEG-MS. (C) 2004 Elsevier B.V. All rights reserved.