One- and two-dimensional NMR imaging techniques have been used to investigate the influence of pore structure on mass transfer through porous alumina catalyst pellets. Studies are reported for two batches of pellets. In the first study it is shown that radial variations in voidage and pore size directly influence mass transfer. In the second study, a relatively homogeneous cylindrical tablet is studied. Effective diffusivity, and hence tortuosity, is observed to differ significantly with respect to axial and radial transient diffusion processes. This observation is correlated with a similar difference in fractal dimension describing the pore structure in these two directions. NMR imaging is shown to be a useful technique for gaining insight into the effect of manufacturing process on the mass transfer properties of porous catalyst pellets.