Nanocrystalline ZSM-5 (similar to 20-50nm) has been tested as a catalyst in the liquid phase rearrangement of 1,2-epoxyoctane, 2-methyl-2,3-epoxybutane and isophorone oxide together with a microcrystalline ZSM-5 sample (similar to 5 mu m) employed as a reference. Whereas this last material shows a low activity for the three epoxide rearrangement reactions considered, nanocrystalline ZSM-5 leads in all cases to high epoxide conversion. This different catalytic performance is attributed to the differences in the accessibility of the reactant molecules to the catalyst active sites. The smallest crystal size of the nanocrystalline zeolite contributes favourably to both the formation of a high external surface area, with no steric constraints, and to a faster intracrystalline diffusion of the epoxide molecules towards the acid sites located within the zeolite micropores. When sterically hindered epoxides are tested, the catalytic rearrangement occurs mainly on the external surface of the nanocrystalline zeolite. Moreover, it is remarkable that for the three epoxides investigated the high catalytic activity observed over nanocrystalline ZSM-5 is accompanied by the attainment of high selectivities, typically around or over 80%, towards products with commercial applications. (C) 2009 Elsevier B.V. All rights reserved.