The effect of precursor on the properties of nanosized sulfide catalysts prepared by the in situ decomposition of nickel-tungsten compounds is studied. Precursors are nickel-thiotungsten complexes [(Ph)(3)S](2)Ni(WS4)(2) in the hydrocarbon feedstock, [BMPip](2)Ni(WS4)(2) in the hydrocarbon feedstock and ionic liquid, and tungsten hexacarbonyl in the hydrocarbon feedstock; oil-soluble salt nickel(II) 2-ethyl hexanoate is used as a source of nickel. The synthesized catalysts are investigated by electron microscopy methods, X-ray powder diffraction, and X-ray photoelectron spectroscopy. Diffusion and chemical reactions in nanosystems are described by the methods of extended irreversible thermodynamics based on a postulate according to which additional variables are time derivatives of usual thermodynamic variables. It is shown that, as the size of nanoparticles decreases, the velocities of diffusion and oxidation chemical reaction in catalyst domains decline; as a result, the content of oxygen in the sample prepared in the hydrocarbon feedstock is lower than that in the sample prepared in the ionic liquid.