Treatment of siliceous MCM-41 with aluminum chloride was performed in a vacuum at a temperature of 403 K. The obtained AlCl3/MCM-41 material was characterized by multinuclear solid-state NMR spectroscopy and by application of probe molecules. The results of NMR spectroscopy indicate a reaction of aluminum chloride with vicinal silanol groups at the surface of conditioned MCM-41 and the formation of tetrahedrally coordinated aluminum species bound to chlorine and framework oxygen atoms. In the local structures of 3% of these aluminum atoms, strong Bronsted acid sites are formed causing a H-1 MAS NMR signal at 6.0 ppm. According to C-13 MAS NMR spectroscopy of acetone-2- C-13 adsorbed on AlCl3/MCM-41 (241 and 245 ppm), these Bronsted acid sites are characterized by an acid strength higher than that of bridging OH groups in H-ZSM-5 (223 ppm).