In a new process of fine tailings disposal, after extraction of bitumen from Athabasca oil sands, the slurry, which contains clays, sand and a small amount of bitumen, is flocculated and thickened into a paste. The effects of pH, polymer dosage and presence of divalent cations on the flocculation of the Syncrude tailings have been studied. The tests were carried out using a high molecular weight polyacrylamide flocculant (Percol 727) over a pH range from 3 to 11. The effect of Mg2+ and Ca2+ ions was also tested. These factors were analyzed by fitting the experimental data to a polynomial model, which plots flocculation response in the form of contour maps. The maps identify the conditions for obtaining high initial settling rates (isr) and/or low solids content (sc) in the supernatant. At low pH, in which clays tend to coagulate, the domains of a high initial settling rate (isr) and low solids content (sc) coincide. In alkaline pH, in which clays form stable suspensions, the addition of the flocculant leads to the development of bimodal size distributions consisting of flocs and dispersed fine particles. Flocculation is much more efficient in the presence of divalent cations over this pH range.