The pKa values of three fluorinated phenols, 2,4,6-trifluorophenol (3FP), 2,3,5,6-tetrafluorophenol (4FP) and 2,3,4,5,6-pentafluorophenol (5FP) have been measured by using UV-vis and 19F-NMR spectroscopy at 25 °C in water and in the presence of pure POPC, pure DDAB and mixed POPC–DDAB liposomes.