We hypothesised that the seemingly contradictory pH-dependent results in the two assays could be related to different rate-limiting steps for H + /Cl -symport and Cl -/NO3 -exchange. As shown by our previous studies, most hydrogen bond-based anion transporters facilitate the transport of the more lipophilic anion NO3 -faster than Cl -as governed by the ease of anion dehydration. However, exceptions have been found for a few transporters, most notably the tripodal tris-(thio)ureas . These transporters are Cl -> NO3 -selective due to their structural mismatch for binding planar NO3 - ions, leading to underestimated Cl -transport activity when tested under the Cl -/NO3 -exchange assay.