Silica-alumina oxide supports were synthesized by surface alumination of mesoporous silica SBA-15 with Al isopropoxide in organic solvents. These materials were characterised by N2 adsorption-desorption isotherms, XRD, 27Al NMR and cumene cracking. The chemical nature of the surface was probed by an original method based on Mo thermal spreading which proves to be very sensitive to the surface properties. The latter vary continuously with Al grafting from pure silica to pure alumina. After three graftings, a well-ordered, alumina-like mesoporous support containing 30 wt% Al2O3 is obtained with a specific surface area of 326 m2.g-1 and a narrow pore size distribution centred on 57 Å.