The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1 mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites (ca. 250 nm). The second sample were laboratory-made large crystals with coffin-like shape (ca. 17 x 4 x 4 mu m(3)). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120 m(2) g(-1) of mesopore surface area, respectively, in contrast to the 62 and 5 m(2) g(-1) of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification. (C) 2011 Elsevier Inc. All rights reserved.