Spatially resolved NMR Imaging techniques have been applied in the study of the three-dimensional structure of aluminium oxide catalyst spheres, since the possible heterogeneities have been found to affect strongly the performance of porous materials as catalysts or catalyst carriers. A special form of activated alumina suitable for Claus catalytic converters was studied, The catalyst spheres were found to exhibit a uniform ball structure with spherically layered structures and voids, ensuring for example optimum flow conditions through the converters.