Bimetallic silver and gold nanoparticles, a core-shell type structure, have been prepared by NaBH4 reduction method. The optical absorption spectra are recorded and compared with various ratios of Ag/Au alloy nanoparticles. The absorption spectra of bimetallic nanoparticles suggested the formation of core-shell structure. On the other hand, the high resolution transmission electron microscopy image of the nanoparticles confirmed the core-shell type configuration directly. Moreover, we studied the surrounding environment of core-shell type and alloy nanoparticles around gold atoms by using extended X-ray absorption fine structure spectroscopy. (c) 2006 Elsevier B.V. All rights reserved.