The reduction of [Ru(COD)(2-methylallyl)(2)] (COD = 1,5-cyclooctadiene) dispersed in various room -temperature ionic liquids (ILs), namely, 1-n-butyl-3-methylimidazolium (BMI) and 1-n-decyl-3-methylimidazolium (DMI), associated with the N-bis(trifluoromethanesulfonyl)imidates (NTf2) and the corresponding tetrafluoroborates (BF4) with hydrogen gas (4 bar) at 50 degrees C leads to well-dispersed immobilized nanoparticles. Transmission electron microscopy (TEM) analysis of the particles dispersed in the ionic liquid shows the presence of [Ru(0)](n), nanoparticles (Ru-NPs) of 2.1-3.5 nm in diameter. Nanoparticles with a smaller mean diameter were obtained in the ILs containing the less coordinating anion (NTf2) than that in the tetrafluoroborate analogues. The ruthenium nanoparticles in ionic liquids were used for liquid-liquid biphasic hydrogenation of arenes under mild reaction conditions (50-90 degrees C and 4 bar). The apparent activation energy of E-A = 42.0 kJ mol(-1) was estimated for the hydrogenation of toluene in the biphasic liquid-liquid system with Ru-NPs/BMI center dot NTf2. TEM analysis of the ionic liquid material after the hydrogenation reactions shows no significant agglomeration of the [Ru(0)](n) nanoparticles. The catalyst ionic liquid phase can be reused several times without a significant loss in catalytic activity.