Mo(W)/SBA-15 catalysts are prepared using heteropoly acids H4SiMo12O40, H4SiW12O40, and H4SiMo3W9O40. The catalysts in the sulfide form are studied by low-temperature nitrogen adsorption, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Catalytic properties are tested in the hydrodesulfurization of 4,6-dimethyldibenzothiophene. It is shown that the gas-phase sulfiding of Mo(W)/SBA-15 catalysts leads to increase in the average length of particles and the number of Mo(W)S2 layers in active phase particles compared with liquid-phase sulfiding with the use of dimethyl sulfide. The replacement of a quarter of tungsten atoms with molybdenum ones makes it possible to considerably improve the catalytic activity of the mixed catalyst Mo + W/SBA-15 compared with the monometallic counterparts. This effect can be enhanced due to the use of mixed heteropoly acid H4SiMo3W9O40 as a precursor of the active phase of the MoW/SBA-15 catalyst, which is apparently associated with the formation of MoWS2 active sites.