The acid-base characteristics of naturally occurring illites, collected from different locations, were investigated by potentiometric titrations. The experimental data were interpreted using the constant capacitance surface complexation model. Considerable release of Al and Si from illite samples and subsequent complexation or precipitation of hydroxyl aluminosilicates generated during the acidimetric forward titration and the alkalimetric back titration, respectively, were observed. Therefore, the acidimetric supernatant, rather than the neutral one, was regarded as the system blank for each illite suspension to yield the surface site concentrations. In order to describe the acid-base chemistry of aqueous illite surfaces, two surface proton-reaction models, introducing the corresponding reactions between the dissolved aluminum species and silicic acid, as well as a surface Al-Si complex on homogeneous illite surface sites, were proposed as follows: (i) Model I, 'SOH N 'SO ؊ ؉ H ؉ , pK a int ؍ 3.76 -4.09, 'SOH ؉ Al 3؉ ؉ H 4 SiO 4 N 'SOAl(OSi(OH) 3 ) ؉ ؉ 2H ؉ , pK SC ؍ 3.06 -3.53; (ii) Model II, Al 3؉ ؉ H 4 SiO 4 ؉ 2H 2 O N Al(OH) 2 (OSi(OH) 3 ) ؉ 3H ؉ , pK f2 ؍ 9.85-10.71, 'SOH N 'SO ؊ ؉ H ؉ , pK a int ؍ 3.73-4.08, 'SOH ؉ Al 3؉ ؉ H 4 SiO 4 N 'SOAl(OSi(OH) 3 ) ؉ ؉ 2H ؉ , pK SC ؍ 3.31-3.48. The K f2 constant in Model II was obtained by simulating the complex formation between the dissolved aluminum species and silicic acid that occurred in acidimetric supernatant when the hydroxide was added. Additionally, the following cation exchange reaction was also considered for a special case, where a large amount of K ؉ is released during the corresponding acidimetric titration, in which a high concentration of protons are consumed. 'XH ؉ K ؉ N 'XK ؉ H ؉ , pK i ؍ 8.24 -8.52 Optimization results indicated that both models could provide a good description of the titration behavior for all aqueous illite systems in this study. The intrinsic acidity constants for the different illites were similar in Model I, showing some generalities in their acid-base properties. Model I may be considered as a simplification of Model II, evident in the similarities between the corresponding constants. In addition, the formation constant for surface Al-Si species (complexes or precipitates) is relatively stable in this study.