Like catanionic vesicles, fatty acid-based vesicles also contain at least two types of surfactants (protonated fatty acid and ionized soap). One important difference between pure fatty acid/soap vesicles (without additional amphiphiles) and catanionic vesicles is that the former contain an amphiphile that is not charged (the neutral form of the fatty acid) and the two bilayerforming components are associated by hydrogen bonds instead of electrostatic interactions [9]. A similar situation holds also for mixed fatty acid-based systems containing a neutral amphiphile at high pH (a linear alcohol or a glycerol ester), together with ionized fatty acid molecules; or negatively charged amphiphilic additives at low pH (e.g. SDBS), together with the neutral form of the fatty acid. However, the difference becomes less clear if a cationic surfactant is added to a fatty acid system. Mixtures of tetradecyltrimethylammonium hydroxide (TTAOH) and fatty acids were investigated as a model system of catanionic vesicles that do not contain additional salt [42•]. Unilamellar and multilamellar vesicles were reported to form, if approximately the same concentration of TTAOH and fatty acid were mixed. So far, fatty acid vesicles and catanionic vesicles were not yet compared systematically, in spite of their apparent similarities.