Potentiometric titration of proton saturated fine fractions of three bentonites were used to characterize the acid centre at the smectite-water interface in dispersions. The H-forms were prepared by H⁺->OH^-->H⁺ ion exchange with resins. The obtained titration data were used in a thermodynamic calculation of proton affinity distribution. Numerical solving of an integral adsorption equation revealed a continuous distribution of proton interaction sites. Proton affinity distributions clearly detected up to 5 different proton interaction sites in all the smectite-water systems, within accessible experimental range of pH’s between 2 and 12. The amount of the strongest acid sites, i.e. those with the lowest pK values, decreased on aging, while the amounts of all weaker acid sites increased with the progress of autotransformation. The strongest acid sites are connected with free protons present in the dispersion, while the weaker acid sites are connected with the titration of released structural Al³⁺, Fe³⁺, Mg²⁺ cations and/or their hydrolyzed species, and deprotonation of SiOH groups. These results indicate the sources of acidity in acid activated bentonites.