Bench-scale experiments were conducted to study the behaviour of oil sands slurries. While a slurry was being stirred with a standard impeller, the mass of bitumen, M-ad, that adhered to a steel probe dipped into the slurry was measured. M-ad remained small up to a critical adhesion time, tau(ad), and then increased rapidly. tau(ad) depended on ore grade, temperature, pH, and clay content. The implication for hydrotransport of oil sands is clear: tau(ad) should be greater than the residence time of the slurry to avoid the problem of bitumen coating the pipe wall and the attendant increase in pumping pressure.