Platinum, palladium and rhodium nanoparticles stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) can be synthesized by ethanolic reduction of the corresponding metal halide salts and when immobilized in an ionic liquid, 1-n-butyl-3-methylimidazolium hexa. uorophosphate ([BMI][PF6]), are very effective olefin hydrogenation catalysts which are capable of being recycled by a simple decantation procedure without loss of activity.