The morphology of silica supported MoO₃ catalysts, which was prepared by impregnation of ammonium heptamolybdate with 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. All morphologies of silica supported MoO₃ appear to be thermodynamically driven. For highly loaded catalysts there appeared three states : a metastable sintered hexagonal state, a well-dispersed hexagonal state at a moderate temperature (300℃), and the sintered orthorhombic state at a high temperature (500℃). Whereas the sintered orthorhombic phase is detected by using XRD at loadings in excess of 1.1 atom Mo/nm², the well-dispersed hexagonal phase is not detected even until 4.0 atoms Mo/nm^{2. 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 higher apparent dispersion of the hexagonal phase may arise from some role of ammonia which results in a stronger MoO₃- SiO₂ surface interaction.}