Mo and Ni-Mo sulfides in SBA-15 and Al-SBA-15 catalysts were prepd. by incipient wetness co-impregnation of molybdenum and nickel precursors (followed by thermal treatment under He at 823 K) and sulfidation under 10% H2S/H2 at 673 K. Only incipient wetness impregnation of Mo on pure silica SBA-15 led to a high dispersion of the MoS2 phase and a high catalytic activity for dibenzothiophene (DBT) hydrodesulfurization (HDS). For the other samples, we obsd. a poor dispersion of the sulfide phase together with a low activity for HDS of DBT (Mo/Al-SBA-15) and 4,6-dimethyl-dibenzothiophene (NiMo/(Al-)SBA-15). In order to rationalize these results, i.e. to understand the reasons for the poor sulfide phase dispersion, we have investigated the chem. phenomena occurring during the process of co-impregnation of Ni and Mo precursors on (Al-)SBA-15. The formation of two Anderson heteropolymolybdates [X(OH)6Mo6O18]n - (X = Al(III) or Ni(II)) during impregnation was demonstrated by XRD and 27Al MAS-NMR. 27Al and 95Mo liq.-state NMR were also used to gain more insight into the formation and resp. stability of these heteropolyanions (HPA) in aq. solns. contg. either Mo and Al, or Mo and Ni, or Mo, Al and Ni precursors. The dispersion of the Mo and NiMo phases in the oxide and the final sulfide phases, were shown to be strongly dependent upon the formation of these Anderson HPA species. [on SciFinder(R)]