To provide fundamental insights into the treatment of oil sand tailings, the forces between a clay fine or silica particle and a silica wafer in aqueous solutions were measured using an atomic force microscope. A high molecular weight, partially hydrolyzed polyacrylamide (HPAM) was used as the flocculant. The effect of polymer dosage, solution pH, and addition of calcium and magnesium ions on the interaction and adhesion forces was studied. Tailings settling tests were carried out to link the measured forces with tailings treatment. The results showed that the addition of the polymer at low dosages or divalent ions at low concentrations resulted in adhesion interactions. The adhesion force increased with increasing polymer dosage or cation concentration until an optimum dosage or concentration was reached. Higher polymer dosage and ion concentration resulted in a weaker adhesion or even purely repulsive force profiles. The synergy of the polymer and divalent ions significantly enhanced the adhesion between fine solids. The measured adhesion forces correlated well with settling characteristics: the stronger the adhesion, the higher the initial settling rate. This study suggests a potential new technology for oil sand tailings treatment using the synergic effect of polymers and divalent cations. (c) 2005 American Institute of Chemical Engineers.