The results show that there is a clear preference for the halide anions over the oxo-anions. The simple spherical geometry of the halide anions makes them a better geometrical fit for the tetralactam binding cavities than the complex geometries of the oxo-anions. Previously, a fluorinated tetraurea macrocycle containing a larger preorganised binding cavity consisting of eight convergent N-H hydrogen bond donors was studied using this assay, revealing the greatest transport selectivity for Cl -. In contrast, the suboptimal size-match between Cl -and the tetralactam cavity prevents the selective transport of Cl -over the other tested anions, leading to a clear selectivity preference for the larger halides instead.