Highly stable palladium nanoparticles 5 nm diam., protected by an imidazolium-based ionic polymer in a nitrile-functionalized ionic liq., have been prepd. and characterized by TEM. These Pd nanoparticles are excellent precatalysts for Suzuki, Heck, and Stille coupling reactions and can be stored without undergoing degrdn. for at ³2 yr. The nanoparticle system may therefore be considered as an alternative to the traditional palladium on carbon precatalyst used in many C-C coupling reactions, and also allowing reactions to be conducted under solvent-free conditions.