The H-2 reduction of RuO2 hydrate "dissolved'' in 1-n-butyl-3-methylimidazolium ionic liquids with different counterions, hexa. uorophosphate (BMI . PF6), tetrafluoroborate (BMI . BF4) and trifluoromethane sulfonate (BMI . SO3CF3), is a simple and reproducible method for the preparation of ruthenium nanoparticles of 2.0 - 2.5 nm diameter size and with a narrow size distribution. The Ru nanoparticles were characterized by TEM and XRD. The isolated Ru nanoparticles are reoxidized in air, whereas they are less prone to oxidation when imbibed in the ionic liquids. These nanoparticles are active catalysts for the solventless or liquid - liquid biphasic hydrogenation of olefins under mild reaction conditions ( 4 atm, 75 degreesC). The catalytic system composed of nanoparticles dispersed in BMI . PF6 ionic liquid is very stable and can be reused several times without any significant loss in the catalytic activity. Total turnover numbers greater than 110 000 ( based on total Ru) or 320 000 ( corrected for exposed Ru atoms) were attained within 80 h for the hydrogenation of 1-hexene.