Clay minerals such as montmorillonite and kaolinite are considered as fine solids that could adversely affect bitumen extraction from oil sands. Colloidal forces between bitumen and fine clays and between bitumen and bitumen in. the presence of montmorillonite clays were measured with an atomic force microscope. Although long-range repulsive forces between bitumen and kaolinite or montmorillonite were similar, stronger adhesion forces were measured between bitumen and montmorillonite than between bitumen and kaolinite, in particular when calcium ions were present. The presence of montmorillonite clays with calcium addition diminished the adhesive forces between bitumen surfaces. Zeta potential distributions of bitumen droplets and clays, individually or in a mixture, were measured in the presence of 1 mM calcium ions. The results showed that kaolinite clays, deposited weakly on the bitumen surface, whereas montmorillonite clays attached strongly to bitumen. Such strong montmorillonite clay attachment set a barrier for bitumen droplets to coagulate with each other and to attach to air bubbles. This behavior accounts for the observed detrimental synergetic effect of montmorillonite clays and calcium ions on bitumen recovery by flotation. (C) 2004 American Institute of Chemical Engineers.