Abstract: A series of M -ZrO₂ composites with various compositions were prepared by co-precipitation with NH4OH from mixed aqueous solution of nitrates and calcination at 500 ℃ and 600 ℃ respectively. The samples were characterized by XRD, XPS, BET, TEM, EXAFS and DTA measurements. The results exhibit that M tends to dissolve into the bulk phase of ZrO₂ to form solid solution as Mg/Zr molar ratio is lower than 1. After the dissolution reaches saturation, M starts to disperse onto the surface of the solid solution, and at the same time the separate crystalline phase of M appears. However, when the content of M is high, ZrO₂ does not dissolve into the bulk phase of M and not form the separate crystalline phase of ZrO₂, but disperses onto the surface of the crystallite of M . And the properties of the surfaces of the samples were obviously affected by the dispersion of ZrO₂. These results suggest that in the samples prepared by co-precipitation, one component can disperse as a monolayer onto the surface of the separate crystalline phase of the other component or the solid solutions composed of the two components in a certain content range. This phenomenon might exist in many binary oxide systems and deserves much more attention.