A Wyoming montmorillonite clay has been intercalated using the competitive ion exchange of aluminum hydroxy polycations and oligosilsesquioxanes. Whereas the intercalation of montmorillonite by the products of hydrolysis of trichlorosilylethylpyridine yields an heterogeneous solid; with a large fraction non-intercalated, the X-ray diffraction shows that competitive intercalation permits a nearly complete intercalation of the material. The lattice spacing of 21.4 Angstrom after ion exchange, is determined by the size of the largest oligosilsesquioxane cations. It is reduced to 17.4 Angstrom after calcination at 873 K where Si-29 and Al-27 MAS-NMR Show the existence of the two types of pillars with respectively the siloxane and Al-13 structure, After calcination at 973 K, a clear 001 XRD line is observed, and the microporous volume is 0.13 ml g(-1), thus showing a good thermal stability. The number and strength of acid sites determined by adsorption of ammonia is comparable to that of HY zeolites. This strong acidity is related in part to the stability of the structure, and to the retention of the individuality of the Al-13 pillars. The tetrahedral aluminum atom of the pillars is assumed to be responsible for this strong acidity.