The sorption of uranyl ions in mesoporous MCM-41 and MCM-48 was accomplished with the help of a direct-template-exchange route, and the progress was monitored as a function of pH of the precursor uranyl nitrate solution. Under identical conditions of synthesis, around one and a half times larger amount of uranium was found to be sorbed in MCM-48 (similar to12.5 wt.%) as compared to MCM-41 (similar to9.5 wt.%). Further, the powder X-ray diffraction (XRD) data revealed that the expansion of unit cell parameters and broadening of reflections of the uranium containing samples depended on the pH of the precursor uranyl solution. Likewise, the Fourier transform infrared spectroscopy (FT-IR) studies showed a progressive decrease in the frequency of the axial O=U=O asymmetric stretching vibrational band, v(a)(U=O) of the anchored uranyl groups with the increase of pH of the exchanging uranyl solution. The presence of two bands at similar to920 and 879 cm(-1) for uranyl exchanged samples prepared at pH > 5 indicated the presence of trinuclear (UO2)(3)(OH)(5)(+) species. The occlusion of uranium thus 5 depends upon the pore structure of the host material and the nature and dimension of the hydrolysis species formed at a particular pH of uranyl solution. Furthermore, the template-exchange of hexavalent uranium in MCM-41 and MCM-48 not only results in the formation of bulky hydrolysis species in the mesovoids, but also substitutes (isomorphously) in the silicate matrix resulting in the formation of UMCM-41 and UMCM-48. (C) 2004 Elsevier Ltd. All rights reserved.