Stable Au-core-Ag-shell bimetallic nanoparticles are produced in organic medium through an interfacial redox method using the redox property of an organically soluble conducting polymer poly(o-methoxyaniline) (POMA). The transition of the emeraldine base (EB) form of POMA to its pernigraniline base (PB) form occurs during nanoparticle formation and also the nitrogen atoms of POMA (1313) stabilize the nanoparticles via coordination. TEM images indicate that the interfacial redox replacement reaction between Au (III) in aqueous medium and Ag (0) in organic medium produces bimetallic nanoparticles with Au-core-Ag-shell morphology. The formation of such type of core-shell morphology is due to the different standard reduction potentials of the metal ions. The core-shell thickness of the bimetallic nanoparticle can be regulated by changing the POMA (EB) concentration in the organic layer. (C) 2009 Elsevier Inc. All rights reserved.