The morphology of silica supported catalysts, prepared by impregnation of ammonium heptamolybdate (AHM) and for various weight loadings up to 35 wr%, was studied using X-ray diffraction. In addition to the orthorhombic phase, the behavior of the rarely studied hexagonal phase was fully characterized. All morphologies of silica supported MoO₃ appear to be thermodynamically driven For high loaded catalysts there appeared three states : a metastable sintered hexagonal state and a well dispersed hexagonal state at moderate temperature calcinations (300℃), and the sintered orthorhombic state at high temperature (500℃). Whereas the sintered orthorhombic phase is detected by XRD at loadings in excess of 1.1 atom Mo/nm², the well dispersed hexagonal phase is not even until 4.0 atoms Mo/nm². It is possible to produce the well dispersed hexagonal phase from the sintered orthorhombic phase with an ammonia impregnation and subsequent calcination at 300℃. The apparently higher dispersion of the hexagonal phase may arise from some role of ammonia which results in a stronger MoO₃-surface interaction.