Zeolites are microporous crystalline materials that find wide application in industry, for example, as catalysts and gas separators, and in our daily life, for example, as adsorbents or as ion exchangers in laundry detergents(1). The tetrahedrally coordinated silicon and aluminium atoms in the zeolite unit cell occupy the so-called crystallographic T-sites. Besides their pore size, the occupation of specific T-sites by the aluminium atoms determines the performance of the zeolites(2). Despite its importance, the distribution of aluminium over the crystallographic T-sites remains one of the most challenging, unresolved issues in zeolite science. Here, we report how to determine unambiguously and directly the distribution of aluminium in zeolites by means of the X-ray standing wave technique(3) using brilliant, focused X-rays from a third-generation synchrotron source. We report in detail the analysis of the aluminium distribution in scolecite, which demonstrates how the aluminium occupancy in zeolites can systematically be determined.