H-1 PFG NMR was performed to measure the diffusion coefficient of methane in three MFI-type siliceous zeolite silicalite samples, which were synthesized under different hydrothermal conditions resulting in different crystal sizes or intracrystalline defects. Measured diffusivities of methane decreased at the diffusion distance around 7 mum for all samples. The distance almost corresponds with the distance between cracks in the intracrystalline. However, the well-calcined sample, which has no detectable cracks as observed by scanning electron microscope, also showed a drastic decrease in the diffusion coefficient. This result suggests the existence of an intracrystalline transport barrier in the single crystal, the size of which is below the SEM resolution. (C) 2004 Elsevier B.V. All rights reserved.