Supported γ-A12O3 membranes are prepared by a dipping procedure. According to this method, a capillary pressure drop is created by bringing a microporous ceramic support into contact with a boehmite (γ-A1OOH) sol. This pressure drop forces the dispersion medium of the sol to flow into the dry part of the support. The sol particles are concentrated at the entrance of the pores and a gel is formed. This gel can be dried and calcined to form a crack-free alumina membrane with pores measuring only a few nanometers, if the membrane thickness after calcination is about 5 μm or less. The gel formation process can be quantitatively well described with the slibcasting model. The observations that the membrane thickness increases linearly with the square root of the dipping time and that the casting rate increases if the sol concentration increases, the pore size of the support decreases or the gelling concentration decreases, are all in accordance with this slibcasting model.