A series of titrations with different anions was carried out for anion free BU 7. In these, BU 7 was first titrated with TBA + CF3SO3 -, monitored by both 1 H and F NMR spectroscopy and the changes in the chemical shifts were analysed by a 1:1 binding model to obtain the Ka of BU 7 for CF3SO3 -. Subsequently, BU 7 was titrated with ReO4 -in the presence of excess CF3SO3 -, enabling the determination a selectivity ratio for the two anions and thus of the Ka of BU 7 for ReO4 -. Further competition studies with ClO4 -and Cl -resulted in the determination of Ka = (4±2)×10 10 M -1 between BU 7 and Cl -. Anion binding data for non-fluorinated BU 12 were obtained using this strategy as well. BU 7 was then used to determine Ka values of BUs 1, 2, 5, 6 and 8-11 towards Cl -in experiments where 7 competed with other BUs for the chloride anions, as analysed using 19 F NMR spectroscopy. In these studies BU 7 was used as an anion-free compound, while the other BU was introduced as either a ~1:1 complex with TBACl or as anion-free compound, in which case a small amount of TBACl was added to the system. The sum of the concentrations of both BUs was larger than that of chloride, ensuring that all chloride was distributed between the two BUs according to their relative ratio in Ka values. This relative difference was then calculated from the areas corresponding to the signals of anion-free and anion-bound BUs. Unfortunately, the Ka values of BUs 3 and 4 could not be obtained as the peaks of anion-free and complexed BU overlapped. A similar problem was encountered for competition experiments with BU 6, for which the peaks corresponding to complexed and anion-free 6 were poorly resolved, allowing only a more rough estimation of its Ka to chloride. The 10 logartim of all Ka values are reported in Table .