H4[SiMonW12−nO40] heteropolyacids (HPAs) are interesting precursors for the preparation of alumina-supported hydrotreatment catalysts to introduce both metals simultaneously while maintaining a Mo–W nanoscale proximity. Two heteropolyacids (n = 1 and 3) have been synthesized and used for the first time to prepare hydrotreatment catalysts. Crystal structure refinement has been performed and evidenced the formation of β-H4[SiMo3W9O40] with three ordered Mo sites forming a face. The purity of the samples in aqueous solution has been determined by Raman spectroscopy and polarographic characterization. These heteropolyacids were then impregnated on alumina to prepare supported MoW-based catalysts. As references, catalysts with the same Mo/W ratios have been prepared using monometallic H4SiMo12O40 and H4SiW12O40 HPAs (mixture of these 2 HPAs in the impregnating solution). EXAFS characterization after drying performed simultaneously at the Mo K and W LIII edges indicates preservation of the mixed heteropolyanion SiMonW12−nO404− at the alumina surface even if partial decomposition to Keggin lacunary species could not be excluded and evidences the mixed MoW–S2 slab formation after sulfidation. Better catalytic hydrogenation properties for dibenzothiophene hydrodesulfurization and naphthalene hydrogenation have been obtained when using β-H4[SiMo3W9O40], which is explained by the formation of the mixed MoW–S2 active phase.