1. From the effect of pH on KCl diffusion potentials an effective titration curve is obtained for the membrane charges controlling ion selectivity in gall-bladder epithelium. The epithelium behaves as if it is negatively charged at physiological pH (K+ and Na+ more permeant than Cl−), has an isoelectric point near pH 3, and is positively charged at lower pH (Cl− more permeant than K+ or Na+). This inversion of selectivity at low pH is completely reversible. 2. 2. The positive charges responsible for selective permeability to Cl− at low pH react with 1, 5-difluoro-2, 4-dinitrobenzene. 3. 3. The relative potencies of 17 polyvalent cations in increasing P Cl/P Na are compared, where P stands for relative permeability coefficients of the ions indicated. 4. 4. Low pH and high Ca 2+ concn. both decrease cation conductance and increase anion conductance. 5. 5. Protonation of membrane charges also affects selectivity among alkali cations: P Cs/P Na inverts, and P K/P Na decreases, at low pH. 6. 6. The results are related to theories analysing inter-cation specificity in terms of the field strength of membrane charged groups.