The binding strength of the BUs to chloride was studied using NMR spectroscopy in CD3CN, with tetrabutylammonium (TBA) serving as the counter cation. Given the strong binding of chloride by BUs 1-12, which precludes the use of a single titration experiment for the determination of the association constant Ka, competition experiments were required. Another complication arises from the difficulties to obtain BUs 2-4 and 9-10 without any anion inside their cavity. However, exchange for another anion was feasible, meaning that the templating HSO4 -anion could be replaced by Cl -.