Hierarchical materials based on crystallization of the MFI structure on mesoporous Al-SBA-15 have been synthesized by a hydrothermal procedure in water. The effect of the crystallization temperature and time as well as the proportion of zeolite template has been investigated. In addition to XRD, TEM and N2 sorption measurements, the structural and textural properties of the materials have also been studied by hyperpolarized (HP) 129Xe NMR and FTIR. These techniques emphasized that the main issue during the synthesis procedure is to find the optimal parameters (time, temperature and concentrations) in order to maintain the structure of the Al-SBA-15 mesoporous solids under the conditions required for zeolite crystallization. Embryonal MFI crystals have been identified on SBA-15 walls which remained ordered for materials obtained with TPABr : Al molar ratio of 0.7 : 1 after 8 h of hydrothermal treatment at 180 1C. Evidence for these zeolite nanocrystals has been obtained for the first time by hyperpolarized 129Xe NMR spectroscopy. Moreover, this was corroborated by the appearance of an IR absorption band at 542 cm1 due to the asymmetric stretching vibration of the double fivemembered rings present in ZSM-5.