Au/Ag alloy nanoparticles consisting of various mole fractions of gold and silver have been synthesized in aqueous medium using a citrate reduction method. UV/vis spectra confirm the formation of alloy nanoparticles and transmission electron microscopy reveals a rather uniform distribution of similar to35 nm-sized particles. The most interesting finding deduced from the surface-enhanced Raman scattering (SERS) of pyridine is that the Ag atoms are highly enriched in the outermost layers of the nanoparticles compared with their interiors. SERS is thus demonstrated to be an efficient surface analyzing tool specifically in nano scales for noble metal alloys. (C) 2004 Elsevier B.V. All rights reserved.