c-Al 2 O 3 materials with small mesopores are hydrophobised by reactions with organochlorosilanes in liquid media. The structure of the modified materials is studied by means of physisorption and high-resolution SEM. A good measure of the decrease in surface polarity can be obtained from BET-fits of the N 2 isotherms. It was found that many pores become blocked by using chlorosilanes with bulky organic groups, while a more open structure is preserved for methylchlorosilanes. Using toluene as a solvent leads to a higher extent of modification than ethanol, as observed for methylchlorosilanes. Multifunctional organochlorosilanes (i.e. with 2 or 3 Cl-groups) are more reactive but also give rise to blockage of some of the microporous part as a result of polymerisation reactions. Mono-and difunctional methylchlorosilanes give the best results for alumina materials with small mesopores, preserving an open structure and leading to homogeneous modification. Modification with methylchlorosilanes in toluene leads to the lowest surface polarity. The results give clues for optimising the procedures for hydrophobisation of porous materials with small mesopores.