According to the pH-partition hypothesis the charged species of organic compounds do not contribute to lipid bilayer permeation as they generally show negligible partitioning into n-octanol. With this assumption, membrane permeation is related to the molar fraction of the neutral species at a particular pH. A recently developed permeation assay permits us to directly determine pH-dependent permeation of aromatic carboxylic acids. Tb(3+)-loaded liposomes are incubated with aromatic carboxylic acids and upon excitation at the absorption wavelength of the acid, permeation kinetics can be measured as an increase in Tb(3+) luminescence. The anions of the tested acids permeated egg phosphatidylcholine membranes only 12 (2-hydroxynicotinic acid), 66 (salicylic acid), and 155 (dipicolinic acid) times slower than the net neutral species. The anions, therefore, controlled the total permeation already at 1-2 pH units above their pK(a). These results indicate that in contrast to the expectations of the pH-partition hypothesis, lipid bilayer permeation of an acidic compound can be completely controlled by the anion at physiological pH.