Grafting a 4-fold alumina multilayer inside the nanombes of a wide-pore MCM-41 material, by consecutive reactions with Al(O-sec-Bu)(3) followed by hydrolysis-calcination, changes the surface chemical functionality of the material from silica to alumina. This procedure leads to partial filling of the MCM-41 mesopores with an amorphous alumina phase. In this publication the multi-grafting process and the structure of the produced aluminum-containing phases are characterized by performing FAM-II enhanced Al-27 3QMAS and 5QMAS NMR experiments. A reference gamma-alumina sample, prepared from Al(O-sec-Bu)(3) by a sol-gel method and calcined at 500 degreesC, with a surface area of 460 m(2)/g and a domain diameter of 1.5-2 nm, showed a similar short range order as commercially obtained well-crystalline gamma-alumina. In the Al2O3/MCM-41 sample grafted in one step (Si/Al = 4.9), Al-species were observed in O-h and T-d positions, belonging to alumina clusters inside the pores (similar to65%), and in T-d sites implanted into the silica framework (similar to35%). After successive grafting (4 steps, Si/Al = 1.6) the relative population of silica-substituted Al sites amounted to about 25%. The aluminum atoms in the grafted phase consisted of O-h (59%), T-d (31%), and stable pentacoordinated sites (10%)-the last at a significantly higher concentration than is generally observed in transition aluminas. This result, together with the large presence of T-d surface sites, can explain the high acidity and catalytic activity of the alumina multilayer inside the pores of the MCM-41.