To understand the pH-dependent Cl -/NO3 -exchange property of the PTSCs, we determined the anion transport selectivity of 1 as a representative example under an HPTS assay utilising a transmembrane anion gradient to induce a pH gradient (Fig. ). In this assay, vesicles containing NaCl (100 mM) and the pH indicator HPTS (1 mM) buffered at pH 7.0 were suspended in NaX (100 mM, X = Cl -, Br -, NO3 -, I -, or ClO4 -) at pH 7.0. Transporter 1 was added to induce a pH gradient where an acidification of the vesicular interiors indicates faster HX influx than HCl efflux (i.e., X -> Cl -selectivity) and vice versa. The assay revealed a selectivity pattern of I -> Br -> Cl -> ClO4 -> NO3 -(Fig. ). Compared with a neutral mono-thiourea transporter showing a Hofmeister-type selectivity pattern (i.e., ClO4 -> I -> NO3 -> Br -> Cl -), the selectivity profile of 1 features a strong halide-over-oxyanion preference with the selectivity sequence within each category (halides or oxyanions) governed by the ease of anion dehydration. This halide selectivity presumably originated from the covalent character of the strong doubly ionic (double charge-assisted) hydrogen bonds in anion binding by protonated PTSCs. As the lone pair orbitals (HOMOs) of the halide ions are of higher energies than those of the tested oxyanions (ClO4 -and NO3 -), charge transfer between the anion HOMO and the antibonding orbital (LUMO) of the hydrogen bond donor is more favourable for halides than for the oxyanions, which accounts for the halide over oxyanion selectivity here observed for doubly ionic hydrogen bonds.