Binary Au-Pd nanoparticles were synthesized by ultrasonic irradiation of solutions containing Au3+ and Pd2+ ions (the ion ratio from 0.3:0.7 to 0.9:0.1 mM) and cationic surfactant (SDS: sodium dodecyl sulfate). In each case the core-shell structure (Au core, Pd shell) was confirmed by scanning transmission electron microscopy (STEM). The mean diameters of them were all about 9 nm, and the thickness of the Pd shell depends on the ratio of Pd2+ and Au3+ ions in solution. In order to study the electronic states of core-shell nanoparticles and their dependence on shell thickness, Doppler broadening measurements were performed for Au-Pd core-shell nanoparticles by using slow positron beamtechnique. The ratio curves of Au-Pd particles did not match with those of pure Pd and pure Au, but a small difference in the low electron momentum region was observed among nanoparticles depending on Pd shell thickness. (C) 2008 Elsevier B.V. All rights reserved.