The study of ion transport across artificial phospholipid membranes by synthetic transporters has become increasingly significant in recent years, as it allows for the emulation of biological membrane functions, including regulation of electrolyte levels and cellular communication. However, most studies have focused on the transport of inorganic anions, whereas the transmembrane transport of organic anions has been given less attention. This is surprising given that organic anions, particularly carboxylates, have essential roles in cells. Carboxylates (RCOO -) are present in biological molecules such as amino acids, peptides, and proteins. Additionally, carboxylates are key molecules in cellular metabolism, biosynthesis, and the release of stored energy through the Krebs cycle. Furthermore, many active pharmaceutical ingredients (APIs), antibacterial, and fungicidal compounds have carboxylate groups in their structure. Despite the relevance of organic carboxylates in biological systems, only a few papers have reported on the assisted transport of different carboxylates or amino acids across artificial phospholipid membranes, while acetate is more often included in anion selectivity studies. These reports give a first indication that the organic moiety significantly influences the physicalchemical properties of the carboxylates and, consequently, their transport. For instance, the organic group affects the hydrophilicity/hydrophobicity balance of the anion, as well as the equilibrium between the anion (RCOO -) and its protonated form (RCOOH), which has fewer restrictions to cross the lipid membrane. Additionally, functional groups in the organic moiety increase the complexity of the process as they can interact with the transporter, 10 lipid membranes, and aqueous medium. These factors make each carboxylate unique, hampering the direct comparison in transmembrane transport experiments. Furthermore, the limited number of carboxylates studied so far limits the understanding of the role of the organic moiety in this process. Herein, we conducted a systematic study into the role of the organic moiety in various carboxylates during their internalization into liposomes. We have examined the size, shape, number of RCOO - groups, and presence of hydroxyl groups in the carboxylate anions (Figure ). We have used various methods to study the spontaneous diffusion of carboxylates, as well as their transport by a reported bisurea-based anion transporter (T1).