Silica and mullite powders have been prepared by the sol-gel method and their surface characteristics studied by inverse gas chromatography (IGC) and zetametry. The optimum conditions were defined using two different catalysts (HCl and 2,4-pentanedione). The dispersive component of the surface energy γ s d of the mullite samples measured by IGC shows a minimum for the molecular ratio Al 2 O 3 SiO 2 = 1.14, indicating high modifications of the surface properties. The ceramic surface is more apolar far from the ideal and theoretical Al 2 O 3 SiO 2 ratio of mullite (1.5), where the apolar component becomes low. Zetametry measurements reveal that a segregation phenomenon of the Si and Al elements takes place at the mullite surface with an enrichment of the surface with silicon during the syntheses. Our results have to be considered when substrates are coated with such alumino-silicates in catalyst preparation.