The force of interaction between two flat double layers with dissimilar Surface potentials but of the same signs, in contact with 1-1 electrolyte solutions (0.001, 0.01. and 0.1 M), was calculated as a function of the Surface separation. The calculation was carried out on the basis of the Gouy-Chapman-Stern-Grahame double-layer model, incorporating the site-dissociation and site-binding model used previously for many oxide studies. The results demonstrated that the interaction force under Surface charge regulation was always repulsive, increasing progressively with decreasing surface separation. This result is in contrast to the predictions of previous theories that are based on the Gouy-Chapman (GC) double-layer model. Dependence of surface potentials and surface charge densities on the surface separation is also presented. (C) 2004 Elsevier Inc. All rights reserved.