To study the efficiency of receptor-mediated transport of non-basic anions (or basic anions whose conjugate acids are not membrane permeable), an assay incorporating the pH-sensitive fluorescent probe 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) encapsulated within synthetic vesicles composed of a single lipid bilayer (unilamellar) using commercial phospholipids such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) may be used. This assay employs HPTS, also known as pyranine, to monitor intravesicular pH, and by using a variety of different conditions, the relationship between anion transport and pH dissipation across the membrane can be determined together with the mechanism of anion transport. Additionally, HPTS is not only useful for monitoring anion transport, but it can also be used as a sensitive vesicle leakage marker. As with other vesicular assays, the HPTS assay relies on the impermeable nature of the lipid bilayer to charged species such as hydrophilic chloride anions that possess significant dehydration penalties.