In this paper, the acid-base properties of illite/water suspensions are examined using the constant capacitance surface complexation model. On the basis of results of potentiometric titrations and solubility experiments, we conclude that the proton reactions in the supernatants of illite suspensions can be successfully represented by proton reactions of Al(H2O)63+ and Si(OH)4 in water solutions. For illustrating the acidic characteristics of aqueous illite surfaces, two surface protonation models are proposed: (1) one site-one pKa model, identical withSOH right arrow over left arrow identical withSO- + H+, pKaint = 4.12-4.23; (2) two sites-two pKas model, identical withSIOH right arrow over left arrow identical withSIO- + H+, pKintaI = 4.17-4.44, and identical withSIIOH right arrow over left arrow identical withSIIO- + H+, [dipKintaII = 6.35-7.74. Evaluation of these two models indicates that both of them can give good descriptions of the experimental data of systems with different illite concentrations and ionic strengths and that the one site-one pKa model can be considered as a simplification of the two sites-two pKas model. Since both models assume only deprotonation reactions at the illite surfaces, they suggest that the surface behavior of the illite is similar to that of amorphous SiO2. Model assumptions, experimental procedures, and evaluative criteria are detailed in the paper.