Ag/Pd nanoparticles stabilized by poly (vinylpyrrolidone) (PVP) have been successfully synthesized in various molar ratios. An ex situ UV-visible spectral analysis was employed to elucidate the growth mechanism of Ag/Pd-PVP nanoparticles, and it revealed that the reduction of Ag ions is much easier than Pd ions in our system. X-ray photoelectron spectroscopy data and mixed potential analysis confirm the effect of the amount of PVP protecting agent to the Ag/Pd-PVP's alloy structure. The growth mechanism of Ag/Pd-PVP nanoparticles is affected by the amount of PVP and forms an inhomogeneous alloy structure. Furthermore, the higher the PVP content, the more inhomogeneous the alloy structure, which is silver-rich in the core and palladium-rich on the surface. The catalytic activity of Ag/Pd-PVP nanoparticles for electroless copper was measured by direct plating on a FR-4 substrate, quartz crystal microgravimetry, and linear scan voltammetry. The performance of Ag/Pd-PVP nanoparticles was also compared with the conventional Sn/Pd activator. Ag/Pd-PVP nanoparticles show the potential for a new activator for electroless copper deposition. (C) 2008 The Electrochemical Society.