Nickel-tungsten sulfide nanocatalysts for the hydrogenation of aromatic hydrocarbons (HCs) have been prepared by the in situ decomposition of nickel thiotungstate precursors in an ionic liquid (IL) using the [BMPip](2)Ni(WS4)(2) nickel thiotungstate complex as a precursor and the thermally stable IL 1-butyl-1-methylpiperidinium trifluoromethanesulfonate BMPipCF(3)SO(3) as a solvent for the precursor. The Ni-W-S particles in situ synthesized in the IL have been characterized by X-ray photoelectron spectroscopy and TEM. It has been shown that the particles are 0.5-nm-wide nanoplates associated in agglomerates with a diameter of 100-150 nm. Catalytic activity has been studied using the example of naphthalene hydrogenation in various solvents in a batch reactor at a temperature of 350A degrees C and a pressure of 5.0 MPa. It has been shown that the IL can be used for the selective hydrogenation of aromatic HCs in a mixture with olefins.