1Department of Applied Chemistry Calcutta University, Calcutta-9 India Potentiometric titration of day poly acidoids prepared by the action of dilute acids on clay fractions isolated from kaolin, aquagel, sedimentary rock, soil, vermiculites and chlorite schist showed two inflections (except in acid montmorillonoid) due to reaction of H+ and Al3+ and polymeric hydroxy Al and Fe with bases. Attempt has been made to show how the titration behaviour, the dissociation constants (pK values) of clay acidoids and the free energy changes in neutralisation reactions are dependent on the chemical composition and charge on unit structural layers. © 1973 Taylor and Francis Group, LLC.