It is evident that the study of diffusion of molecules confined in microporous material, such as zeolites, gives valuable information about the chemical and geometrical properties of the pores. However, due to the limited size of the microporous crystallites or grains, and a non-zero observation time, it is difficult to measure true intra-crystalline diffusivity directly. The measured value of the intra-crystalline diffusivity as derived from PFG-NMR measurements turns out to be dependent on the observation time even if this is very short. This difficulty is inherent to the PFG-NMR technique and due to the limited size of microporous crystallites. Here we present NMR diffusion measurements of ethane confined in H-ZSM-5 crystallites where the mean crystallite diameter was of the order of 20 mm prior to the sample preparation. We apply the short diffusion time model and extrapolate to zero observation time to obtain the unrestricted intra-crystalline diffusivity. The root-mean-square displacement in the diffusion measurements is then only a small fraction of the mean crystallite diameter. We will also describe pitfalls which are present when analysing the experimental data sets.