Colloidal stability of two latexes with variable charge surface groups has been studied at a range of pH. Experimental log W vs log[electrolyte] plots were fitted using the DLVO theory, and values for the diffuse potential, psi(d) and the Hamaker constant, A, were obtained. Reasonable tendency was found for the diffuse potential of both latexes, decreasing when the surface charge decreased. However, the Hamaker constant, which was expected to be constant over the range of pH covered, also showed a decreasing behavior with decreasing charge. Several theories for the electric repulsive potential have been used to try to explain this paradoxing trend, although none of them was successful. Finally, two explanations are proposed, one based on colon adsorption and the other on the hairy layer model, although the first one seems to be the most probable on the basis of thermodynamics considerations.