When the anion transport selectivity sequence remains ambiguous after being subjected to the anion gradient assay (e.g., the maximum ΔpHIN values are similar for anions X -and Y -), an adjusted 'anion gradient' assay, also referred to as the 'competitive' assay, has shown to aid in quantitative determination (Figure ). The 'competitive' assay utilises similar conditions to the 'anion gradient' assay; however, a NaX IN /NaY OUT concentration gradient rather than NaCl IN /NaX OUT is used for a direct and more sensitive comparison between X -and Y -. Therefore, a combinational approach using both the 'anion gradient' and the 'competitive' HPTS transport assays is recommended to prevent misinterpretation of data. Herein we report the required equipment, appropriate methodologies and troubleshooting guidance for conducting the HPTS transport assay.