Abstract: A series of ZrO2-Al2O3 composites with various compositions were synthesized through heat decomposition of metal hydroxides, which were prepared by co-precipitation with NH4OH from aqueous solution of nitrates. The samples were characterized by XRD, XPS and BET measurements. It was found that with content increasing, one component tended to dissolve into the bulk phase of the other to form solid solution first. After the dissolution reached saturation, a component started to disperse onto the surface of the crystallite of the solid solution of the other component. The formation of the separate crystalline phase of one component was observed only when its content was higher than a certain percentage. The XRD peaks of the composites were significantly broadened, and the BET specific surface areas of the composites were larger than those of the pure oxides, which suggested that the crystallites became smaller. The experimental results showed the existence of mutual dispersion of both components in this system. It means that in a certain content range, the crystallite of the solid solution of each component can accept the dispersion of the other component. The phenomenon of mutual dispersion and formation of solid solution would make the crystallites smaller, and lead to higher surface area, since the process could hinder the interface diffusion and crystallite sintering.