New mixed H 4 [SiMo n W n-12 O 40 ] (n = 6 and 9) Keggin type heteropolyacids (HPAs) have been successfully synthesized, as confirmed by single-crystal XRD, Raman and IR spectroscopy analysis. The resulting polyoxometallates were used for preparation of hydrotreatment catalysts. Mo(W)/Al 2 O 3 catalysts were synthesized by incipient wetness impregnation of alumina support with water solutions of prepared mixed Keggin HPAs and corresponding counterparts based on mixture of monometallic H 4 [SiMo 12 O 40 ] and H 4 [SiW 12 O 40 ] HPAs. Oxidic catalysts were analyzed by Raman spectroscopy to determine the precursor structure after deposition. Catalysts in sulfided state were characterized by high-resolution transmission electron microscopy (HRTEM), high angle annular dark field imaging (HAADF) and X-ray photoelectron spectroscopy (XPS) and were tested in co-hydrotreating of dibenzothiophene (DBT) and naphthalene. The use of new mixed Keggin HPAs made it possible to obtain catalysts with mixed MoWS 2 active centers, which was confirmed by HAADF. Moreover, the Mo/(Mo + W) ratio has a direct effect on the structure of the active phase species. An ordered core-shell structure with Mo atoms in the core is maintained until the fraction of molybdenum in mixed MoW/Al 2 O 3 catalyst exceeds 50 %, where a more disordered structure is observed. Moreover, this Mo/(Mo + W) ratio of 0.5 is optimal to achieve a maximum catalytic activity. Indeed, the turnover frequencies (TOF) of the MoWS 2 edge centers with random atoms distribution in a cluster as in Mo 9 W 3 /Al 2 O 3 , was lower compared to that of Mo 6 W 6 /Al 2 O 3 with core-shell structure.