An atomic force microscope has been used to measure the force of interaction between a SiO2 glass sphere (similar to 5 mu m diameter) and a TiO2 crystal, in an aqueous medium over the pH range 3-9. The force-separation profiles obtained were compared with DLVO theory, which was found to adequately account for the experimental results up to separations of ca. 2 nm. At distances below this point repulsive interactions in excess of DLVO may be present under certain conditions. To complement the diffuse layer potential results extracted from the force-separation data for dissimilar materials, force-separation interactions were also measured between two SiO2 glass spheres. For both sets of surfaces, modeling of the experimental force-separation curves was best achieved using constant-charge rather than constant-potential boundary conditions.