X-ray diffraction (XRD), ²⁷Al magic-angle-spinning (MAS) NMR, N₂ adsorption measurements and transmission electron microscopy (TEM) show that the one-dimensional channels of the mesoporous aluminosilicate MCM-41 containing only structural (four-coordinate) aluminium are much shorter in the sample with an Si/Al ratio of 34.1 (ca. 160 Å) than in purely siliceous MCM-41 (ca. 400 Å). The quality of the XRD patterns rapidly deteriorates as the aluminium content of the solid increases. However, aluminosilicate MCM-41 still preserves a perfect mesoporous structure with an average pore diameter of ca. 30 Å, a surface area of 990 m² g^–1 and a pore volume of 0.74 cm³ g^–1. When the Na⁺ form of aluminosilicate MCM-41 is transformed into the Brønsted acidic H⁺ form, and some of the aluminium is removed from the structure, the uniform hexagonal mesopores partially collapse and macropores 200–2000 Å in diameter are formed.