In this work, the physicochemical phenomena occurring during equilibrium impregnation of Anderson-like decamolybdocobaltate H4Co2Mo10O386- heteropolyanion aqueous solutions over gamma-Al2O3 were described in detail comprising chemical analysis, pH measurements, Raman, and UV-vis spectra. For a surface density lower than 2.5 Mo atoms nm(-2), the buffering effect of the support leads to decomposition of H4Co2Mo10O386- into monomolybdates MoO42- and Co2+ cobalt cations that are then adsorbed by electrostatic and covalent interactions with gamma-alumina. Between 2.5 and 3.8 Mo atoms nm(-2), MoO42- monomers condense into heptamolybdates Mo7O246- that are then adsorbed by electrostatic interactions and H4Co2Mo10O386- becomes stable because of the lowering of the pH. Above 3.8 Mo atoms nm(-2), the quantities of adsorbed MoO42- and Mo7O246- become much smaller than that of electrostatically adsorbed H4Co2Mo10O386-. Adsorption of preserved H4Co2Mo10O386- could be consecutive to the decomposition of the first molecules leading to prior adsorption of MoO42- and Co2+, and decrease in the buffering effect of gamma-Al2O3 and in the pH value. For dry impregnation, the same physicochemical phenomena occur considering a given Mo surface density. The methodology used in this work to rationalize the preparation of hydrotreatment catalysts from H4Co2Mo10O386- heteropolyanions can be transposed to any supported catalyst.