Surface species formed upon impregnation of silica, alumina and titania with aqueous solutions of ammonium molybdate and calcination at 770 K have been studied by X-ray diffraction, specific surface area assessment, scanning electron microscopy, laser Raman spectroscopy and temperature-programmed reduction. Dispersion decreases in the order TiO₂> Al₂O₃SiO₂. Polymolybdate species are formed for low loadings, that lead to bulk MoO₃ for loadings of 0.4 monolayer on SiO₂, but persist for higher loadings on titania and alumina. On this last support, isolated molybdate species have been also found.