Surface-active chemicals (e.g., demulsifiers) are frequently used to destabilize crude oil emulsion. It was observed that, in some instances, when mineral solids were present, heavy oil emulsions were difficult to treat. Using three-phase contact angle measurements and videomicroscopy, the reasons behind the ineffectiveness of some demulsifiers were determined. It was found that adsorption of the surfactant onto solids changes the wettability of the solids, and promotes adhesion of the oil droplets to the solids thus reducing the effective density difference between the oil and the water, and hindering oil-water separation.