Suspensions of thirty six bentonites were equilibrated for 24 h and then adjusted to pH 3. After an hour, pH titrations up to pH 12 at varying ionic strengths were performed within 4.5 h. The titration data were recalculated into proton affinity distributions (PAD) using the condensed approximation. In spite of widely varying chemical compositions, all bentonites showed a protonation reaction between pK 4 and 5, which could be assigned to aluminol groups at the edge surfaces of the smectites. The average pK is 4.7380.31. A second prominent peak in the PAD at pK = 10.4380.13 could be assigned to exchangeable Mg2+ in the interlayer space. The possibility to constrain both pK values at a fixed average value will decrease the number of parameters in surface complexation modelling and thus enhance its convergence.