MoW catalysts supported on Al2O3 with equal surface content of metals (Mo + W = 3.9 at/nm2) were synthesized by using mixed Keggin type heteropolyacids (HPAs) H4[SiMo1W11O40] and H4[SiMo3W9O40] and corresponding mixture of monometallic H4[SiMo12O40] and H4[SiW12O40] HPAs. After liquid phase sulfidation, catalysts were characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). For the first time, High Angle Annular Dark Field microscopy (HAADF) was used to evidence the morphology and composition of the active sulfide phase. The catalysts were tested in hydrotreating of model feed that contained dibenzothiophene (DBT) and naphthalene. Using mixed SiMonW12-nHPAs as starting precursors had a beneficial effect on the catalytic activity. Incorporation of molybdenum in the structure of SiW Keggin-type HPA led to a decrease of the size of active phase crystallites, to an increase of the metal sulfidation degree and of the number of active sites. Substitution of a quarter of tungsten atoms by molybdenum allowed to achieve DBT conversion comparable to value obtained on pure SiMo12HPA based catalyst and higher conversion of naphthalene. Both SiMonW12-nHPAs based catalysts had higher rate constants in studied reactions compared to their corresponding references prepared from two separate monometallic HPAs. Moreover, MonW12-n/Al2O3 catalysts had the highest selectivity in respect of hydrogenation (HYD) pathway of DBT hydrodesulfurization (HDS). The presence of mixed MoxW1-xS2 slabs was evidenced by HAADF analysis. On Mo3W9/Al2O3, small randomly distributed islands of Mo were present in the WS2 slabs, while on Mo3 + W9/Al2O3, Mo islands appeared larger and in the core of the WS2 slabs. Moreover, on this last sample, some monometallic slabs were also present. It was concluded that using mixed HPA precursors resulted in the formation of mixed MoxW1-xS2 active phase possessing higher synergetic effect between the two metals and consequently higher catalytic activity.