This paper presents a study of MFI-type zeolite crystal growth during hydrothermal synthesis of nanocomposite MFI-alumina membranes by the pore-plugging method, using the standard protocol described in a previous study [S. Miachon, E. Landrivon, M. Aouine, Y. Sun, I. Kumakiri, Y. Li, O. Pachtova Prokopova, N. Guilhaume, A. Giroir-Fendler, H. Mozzanega, J.-A. Dalmon, Nanocomposite MFI-alumina membranes via pore-plugging synthesis: specific transport and separation properties, J. Membr. Sci. 281 (2006) 228]. To this aim, the materials have been characterized by SEMI, EDX, pure H2 gas permeance and n-butane/H2 mixture separation at different stages of the synthesis. The effect of synthesis time in the range of 4-89h and the effect of a 9-h interruption after a 8-h hydrothermal synthesis have been surveyed, as well as the mean pore size and the alumina phase of the support inner layer. Our results suggest that an interruption during the synthesis is necessary to allow the zeolite precursor to diffuse into the Support pores. This diffusion leads to a further growth of zeolite crystals into the support matrix without formation of a continuous zeolite film on top of the support, as is usually reported in the literature. The zeolite crystals are fully embedded into the support top layer after at least 53-h synthesis time. leading to high quality membranes in only one synthesis run. The nanocomposite MFI-alumina architecture at the nanoscale has important consequences in improving the gas separation performance of this kind of materials when compared to more conventional film-like structures. A method based on gas transport measurements has been used to determine the effective thickness of the separating material. (C) 2008 Elsevier B.V. All rights reserved.