The electrokinetic potential (zeta-potential) is calculated using the combination of the Poisson-Boltzmann equation and the Surface Complexation Models for energetically heterogeneous surface. This approach gives negative values of for the whole pH region, which is in agreement with the published results of zeta-potential measurements. Moreover, the correct changes of zeta-potential values for different types of montmorillonite are observed. It is shown that electrokinetic properties of the considered systems are not as sensitive to surface heterogeneity as the adsorption isotherms and surface charge densities. As it was shown in our previous paper [P. Zarzycki, F. Thomas, J. Colloid Interface Sci. 302 (2006) 547-559], the presented model predicts parallel titration curves for reasonably small equilibrium constants of exchange processes. Here, we show further development of this model in order to predict zeta-potential. (c) 2007 Elsevier B.V. All rights reserved.