An essential first step in most techniques for using anomalous‐scattering data for phase determination is to determine the positions of the anomalous scatterers. This is usually done by use of the anomalous differences, either as input to a direct‐methods procedure or to produce a Patterson map. If the arrangement of anomalous scatterers is noncentrosymmetric then it is also necessary to find their absolute configuration and a process is described for doing this based on the properties of thePsfunction [Okaya, Saito&Pepinsky (1955).Phys. Rev.98, 1857–1858]. If the arrangement of anomalous scatterers is centrosymmetric then the problem does not