The monolayer dispersion of molybdenum oxide on the surface of α-Al2O3 has been studied by X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS). The dispersion threshold is 4.65 mg MoO3 per g α-Al2O3 or 5.6 Mo atoms nm−2. The structure of supported MoO3 is strongly dependent on the MoO3 loading. When the amount of MoO3 is below the dispersion threshold, the MoO3 species are highly dispersed on the surface of α-Al2O3, with the dispersed phase consisting of discrete [MoO4] polyhedrons. When the MoO3 loading exceeds the dispersion threshold, crystalline MoO3 appears on the surface of α-Al2O3. MoO3 exists in both dispersed and crystalline phases. According to the theory of phase isolation, the dispersed phase remains in the form of discrete [MoO4] polyhedrons. However, the crystalline phase of MoO3 is octahedral.