The distribution of framework aluminum inside micrometer-size particles of the mesoporous molecular sieve MCM-41 has been investigated using transmission electron microscopic (TEM) images, after supporting platinum clusters inside the aluminum-containing region selectively via the ion exchange of Pt(NH3)(4)(2+). The TEM images show that the framework aluminum becomes distributed homogeneously inside the resultant MCM-41 particles if sodium aluminate is added to the reaction mixture, consisting of an aqueous solution of hexadecyltrimethylammonium (HTA) chloride and sodium silicate, before the formation of the surfactant-silicate mesostructure. On the other hand, the framework aluminum can be incorporated into the outer region within MCM-41 particles if the aluminum source is added after completing the formation of the mesostructure. The thickness of the aluminum-containing region, which results from the aluminum diffusion into the surfactant-silicate mesostructure, can be controlled by the diffusion time and the concentration of the aluminum source. In addition, powder X-ray diffraction indicates that the structural order is unproved when the aluminum source is added after the formation of the surfactant-silicate mesostructure, rather than before.