Mesoporous aluminosilicate molecular sieves with the MCM-48 structure and atomic Si/Al ratios of 15-40 have been synthesized using aluminum chloride hexahydrate and aluminum sulfate as the sources of aluminum. The products were characterized by powder X-ray diffraction (XRD), thermal analysis (TG/DTA), diffuse reflectance Fourier-transform infrared (DRIFT), Si-29 and Al-27 magic-angle-spinning (MAS) NMR, and nitrogen sorption. The quality of the product rapidly deteriorates as the aluminum content of the solid increases. The walls of the mesopores are essentially amorphous, and the local atomic arrangement is similar to that in amorphous aluminosilicates. The regular arrangement of uniform channels is the only element of order. Al-27 MAS NMR shows that when aluminum sulfate is used, both 4- and 6-coordinate aluminum is present in the calcined products. On the other hand, when aluminum chloride hexahydrate is used, aluminosilicate MCM-48 can be prepared with all aluminum in 4-coordination and the Si/Al ratio as low as 15. XRD shows that the substitution of the silicon by the larger aluminum atoms leads to a contraction of the unit cell.