Potentiometric titration behaviour of complex illitic clay minerals, provided from different origins (two Tunisian illite samples and an American illite sample), were investigated and interpreted according to surface complexation theory. In the present investigation, the focus was on the surface charge characteristics. Proton surface charge can be calculated by subtracting supernatant titration curves from those of illite suspension at ambient temperature and aerated medium. The points of zero charge (PZC), determined by Gran plot method and fast titration technique, were in the range ~8.5-9.2. Our potentiometric titration curves were modelled using derived parameters of Gran method. The surface ionization constants were determined by implementing the theory of surface complexation models (SCMs): the NEM, the constant capacitance model (CCM) and the triple layer model (TLM). The pKas values determined from derived parameters of Gran plot data were in the range (pKa1= 4.5-5.52 and pKa2 =9.78-10.4) and are compared to those obtained by SCMs.