Poly (N-vinyl-2-pyrrollidone) (PVP) has been widely used as a stabilizer to synthesize various kinds of nanoparticles. However, the effect of PVP and its role in the formation process of Ag/Pd nanoparticles are not well understood. In our PVP-protected Ag/Pd nanoparticles system, Ag+ ions have higher priority to be reduced in the formation process of Ag/Pd nanoparticles. The presence of PVP hinders both Pd2+ and Ag+ ions. However, the steric obstacle effect of PVP is more pronounced for Pd2+ ions and thus Pd2+ ions are more difficult to diffuse through the polymeric barrier in the particle growth stage. Accordingly, Pd2+ ions deposit onto the preformed nuclei later than Ag+ ions rendering the palladium-rich surface structure of Ag/Pd nanoparticles. Moreover, the steric barrier of PVP is less effective with increasing molecular weight, possibly owing to the decreasing molecular numbers. The feasibility of Ag/Pd nanoparticles as activators for electroless copper deposition was also carried out. The crystalline structure of electroless Cu film was not affected by the manner of activation. Furthermore, the Ag/Pd-(3/7) nanoparticles had the shortest induction period and thus exhibited the highest activity, which demonstrated their potential to be novel activators for electroless deposition. (c) 2006 The Electrochemical Society.