In summary, we have developed an expanded class of phenylthiosemicarbazones as pH-switchable Cl -/NO3 -exchangers with supressed activity at pH 7.2 and enhanced activity at pH 4.5. We showed that these transporters are active H + /Cl -symporters and inactive Cl -uniporters at pH 4.5 similar to the behaviour of prodigiosin. Most importantly, we demonstrate the strong halide over oxyanion transport selectivity of the phenylthiosemicarbazones presumably due to the covalency of the doubly ionic hydrogen bond interactions leading to the preference for halide ions over oxyanions with low HOMO energies. As a result, while Cl -transport was efficient at both pH 4.5 and pH 7.2, the slow NO3 -transport turned out to be the rate-limiting factor for the suppressed Cl -/NO3 -exchange at pH 7.2 and the activation at pH 4.5. Our results provide important insights for understanding the behaviour and improving the robustness of pH-dependent anion transporters aimed at therapeutic applications. In particular, the study demonstrates the limitation of the anion exchange assay and the benefit of measuring pH-dependent H + /Cl -symport and Cl -uniport activities when studying pHswitchable transporters.