UV-Vis spectroscopy was used to determine the affinity constants of macrocycles 1 and 2 with TBAH2PO4 and TBA2H2P2O7 in DMSO. Titrations with TBACl were performed for comparative purposes and fitting with a 1:1 binding model gave affinity constants in agreement with those obtained by the 1 H-NMR titrations (see Table and Figures ). The data from the UV-Vis spectroscopy titrations of 1 with H2PO4 -and H2P2O7 2-could be fitted with a 1:1 binding model and resulted in affinity constants of 8.9•10 2 and 3.9•10 3 M -1 respectively (Figures ). The UV-Vis spectra from the titration of 2 with H2PO4 -show no isosbestic point (Figure and) and gave very poor fits when using a 1:1 binding model, while a much better fit was obtained when using a 1:2 binding model (Figure , inset), yielding large Ka values for both the 1:1 and 1:2 host to guest association processes (1•10 6 and 5•10 5 M -1 ). The formation of a 2•(H2PO4 -)2 complex is in agreement with the findings from the 1 H NMR titration (Figure ) as well as the crystal structure (vide infra). In the case of the titration of 2 with H2P2O7 2-, an isosbestic point was observed and the data could be fitted to a 1:1 binding model (Figure and S18), giving a K1:1 of 6•10 6 M -1 . During the 1 H NMR titration, the formation of a 2:1 host-guest complex was observed (at 1 mM), but the 200fold lower concentration of 2 (5 μM) would decrease the fraction of this complex present in solution.