Magnesia-silica mixed oxides were prepared by the sol-gel method. Simultaneous hydrolysis of magnesium diethoxide and tetraethoxysilane was made at pH 3 and 9. The solids obtained in acid medium show large surface areas between 441 and 522 m(2)/g, and those prepared in basic medium show surface areas between 132 and 174 m(2)/g. The solids show acidity and basicity, and they are able to retain ammonia and CO2 at 200 degrees C. They can develop basicity by contact with a sulfuric acid solution. Sulfation of the solids notably increases the basicity. The basicity of sulfated oxides is explained in terms of a magnesia segregation. Sulfation segregates magnesium oxide, and hence well-dispersed magnesia on silica is obtained.