Abstract The technique of small angle X-ray scattering (SAXS) has been employed to monitor the influence of uranium substitution on the pore characteristics of MCM-48 silica mesoporous material. The incorporation of uranium, as uranyl ion, at the framework silica sites resulted in the progressive increase in average pore size, the extent of which depended upon uranium loading. These results are in agreement with powder XRD results, showing a progressive increase in unit cell constant ( a 0) with increasing U-loading. The changes in the pore structure and the unit cell parameter are attributed to the perturbations arising due to bonding of (O U O) 2+ units to Si–O − units, thus forming a local uranate type structure. However, the increase in uranium content beyond 3 wt.% resulted in partial precipitation of uranium oxide crystallites, thus causing blocking/narrowing of some of the pores in the host material. SAXS results reveal that, in spite of these structural changes, the fractal nature of the host material remained by-and-large unchanged. These findings are in harmony with the results of the parallel studies conducted using N 2-adsorption and TEM studies.