A method for synthesizing unsupported nickel–tungsten sulfide hydrodearomatization catalysts by breaking SPAN-80 surfactant-stabilized nonaqueous emulsions of solutions of different precursors in dimethyl sulfoxide (DMSO) in situ in a hydrocarbon medium has been first studied using ammonium thiotungstate (NH4)2WS4 and 1-butyl-1-methylpiperidinium nickel thiotungstate [BMPip]2Ni[WS4]2 as precursors and nickel nitrate hexahydrate as a nickel source. The synthesized nickel–tungsten catalysts have been characterized by TEM and XPS. The catalytic activity of the in situ synthesized Ni–W particles in naphthalene hydrogenation has been studied at temperatures of 350–400°C and a hydrogen pressure of 5.0 MPa.