A range of mesoporous aluminosilicate molecular sieves with the MCM-41 structure has been synthesized using different sources of aluminum and characterized in detail by powder X-ray diffraction (XRD), Si-29 and Al-27 magic-angle-spinning (MAS) NMR, and transmission electron microscopy (TEM). NMR clearly shows that, when Catapal alumina or sodium aluminate is used, virtually all Al in the solid is 6-coordinate. On the other hand, by using aluminum sulfate, MCM-41 can be easily prepared with all aluminum in 4-coordination and a framework Si/Al ratio as low as 10. Resolution of the XRD pattern of aluminosilicate MCM-41 rapidly deteriorates as the aluminum content of the solid increases, and TEM shows that the mesopore system is no longer strictly hexagonal. On the other hand, the resolution of the XRD patterns improves upon calcination of the samples at 550 degrees C for 24 h, indicating that heat treatment enhances the formation of the mesopores. The walls of the mesopores are essentially amorphous, and the local atomic arrangement is similar to that in amorphous aluminosilicates.