Room temperature imidazolium-based ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) have been used as effective liquid media for the synthesis of pure Au and bimetallic PdAu nanoparticles by direct synthesis and phase-transfer methods. The mode-of-stability, long-term stability, and long lifetimes of these ionic-liquid supported nanoparticle catalysts, all of which are important factors in determining the overall "greenness" of such materials, were investigated. Four different stabilizing systems in BMIMPF6 ILs were investigated: poly(vinylpyrrolidone) (PVP), 1-methylimidazole, 1-(2'-aminoethyl)-3-methylimidazolium hexafluorophosphate, and pure BMIMPF6 IL with the absence of a secondary stabilizer. The stability of pure Au nanoparticles synthesized by the above four stabilizers was studied using UV-vis spectroscopy and transmission electron microscopy (TEM). It was found that PVP-stabilized nanoparticles were the most stable to aggregation. The catalytic activity of the resulting PdAu nanoparticles was examined for the hydrogenation of 1,3-cyclooctadiene and 3-buten-1-ol across all of the systems to understand which stabilizer(s) are most optimal for nanoparticle catalyst synthesis and usage; particularly which systems have high catalytic activity and selectivity as well as long catalyst lifetimes. In agreement with Au nanoparticle stability results, PVP-stabilized PdAu nanoparticles were the most catalytically active due to improved nanoparticle stability, followed by nanoparticles stabilized by 1-methylimidazole, amine-functionalized IL and the pure BMIMPF6 IL (C) 2010 Elsevier B.V. All rights reserved.