Silica– and alumina–carbon composite xerogels were obtained by the sol–gel method and carbonization of the product obtained at 773 and 1173 K in inert atmosphere. Samples so obtained were characterized by CO₂ adsorption at 273 K and by mercury porosimetry, X-ray diffraction and scanning electron microscopy. With these techniques the surface area and porosity of the samples were determined, as well as the nature of the inorganic component of the composite and the morphology of the surface. The acid–base surface characteristics of the samples were studied by the decomposition reaction of isopropanol. The Al₂O₃-containing samples had more developed meso- and macroporosity than the SiO₂-containing ones. The surface areas obtained for the carbonized derivatives varied between 300 and 400 m² g⁻¹. Alumina–carbon composite xerogels were much more acidic than the silica–carbon composite xerogels. Due to this difference, the electronic interaction between the carbonaceous component, with basic character, and the alumina in the composite xerogels caused a decrease in the total surface acidity of the composite. This electronic interaction increased with the treatment temperature.