Diffuse reflectance spectroscopy (DRS) has been used to investigate structural modification of aluminum-incorporated mesoporous molecular sieves MCM-41. A room temperature synthesis method was used for preparation of pure silica MCM-41 and for aluminum-containing AIMCM-41 materials. Samples with Si/Al ratios of 100, 50, 25 and 10 were synthesized. A post-synthesis modification of a purely siliceous MCM-41 was also carried out using an aqueous aluminum sulfate solution with appropriate concentration to obtain materials with the same Si/Al ratios. The products possess highly ordered structure and high surface area as evidenced from X-ray diffraction (XRD) and BET measurements, respectively. DR spectra of aluminum-containing materials show a distinct and well-defined band at = 200 nm correlated to framework aluminum. The intensity of this band is proportional to the amount of aluminum incorporated. post-synthesis-modified materials show a slightly lower incorporation of aluminum according to the DR spectra and energy-dispersive X-ray (EDX) analysis. Reflectance spectra of the as-synthesized samples, irrespective of the preparation method, show no significant amount of extra-framework aluminum. A very small amount of extra-framework aluminum was found when samples with a low SUM were calcined. These species are detected by producing a broad band at about 240-280 nin in the DR spectra. (C) 2004 Elsevier B.V. All rights reserved.