Mercaptopropionic acid (MPA) capped 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-visible spectra confirm the formation of alloy phases and transmission electron microscopy (TEM) reveals a rather uniform distribution of several nanometer-sized particles. The particles could easily be phase-transferred into organic solvents, isolated, and redispersed in a variety of solvents such as benzene, chloroform, and dichloromethane. The phase transfer from aqueous medium to an organic solvent does not alter the particle size and distribution. It is also recognized that the surfactant, alkaline MPA, should be aged for about 3 days in order to observe the formation of silver and silver-rich phases of alloy nanoparticles. The reason for this observation is discussed in terms of the competitive interaction between the Na+ and -COOH/-SH groups and subsequent reduction of metal ions. (C) 2004 Elsevier Inc. All rights reserved.