The morphology of silica supported MoO3 catalysts, which was prepared by impregnation of ammonium heptamolybdate wich various weight loadings up to 35 wt%, was studied using x-ray diffraction. In addition to the orthorhombic phase, the behavior of the rarely studied hexagonal phase was characterized. For high loading catalysts, excess ammonium ions present in the monoclinic and triclinic precursors are capable of cooupying interstibial sites of microcrystalline MoO3 during moderate temperature calcinations and in doing so enhance the MoO3-SiO2 interaction. This results in a "well dispersed" morphology at high loadings. Sintering at high temperature is due to loss of ammonium from the oxide framework. Ammonia reimpregnation, which leads back to the well dispersed hexagonal phase, may offer a simple regeneration process for spent Mo containing catalysts.