The nature and concentration of the acid sites of MCM-41 material as a function of Si/Al ratio have been monitored by in situ IR using pyridine as a probe molecule. The thermal stability of aluminium in the framework of these materials is studied using Al-27 MAS NMR spectroscopy. The tetrahedral to octahedral aluminium ratio is found to increase with higher aluminium incorporation. The sample with the lowest Si/Al ratio is found to exhibit higher acidity compared to the other samples. However, the acidity of these mesoporous aluminosilicates is less compared to the conventional H-Y zeolite.