This paper reports the internal structure of Ag-Au bimetallic nanoparticles with hollow interiors and alloyed shells, synthesized by chemical reduction of metallic precursor and subsequent galvanic replacement reaction. By taking advantage of advanced electron microscopy methods such as high-resolution transmission electron microscopy (HRTEM), scanning TEM (STEM), weak beam dark-field (WBDF) microscopy, X-ray energy dispersive spectroscopy (EDS) and nano-beam electron diffraction (NBD), sufficient and necessary evidences confirm the sacrificial role of silver nanoparticles as templates for the epitaxial deposition of gold-rich atomic layers. (C) 2009 Published by Elsevier B.V.