Studies of the processibility of a variety of types of Athabasca oil sands have been carried out to further elucidate the physical action of natural surfactants in the hot-water flotation process. In particular, the interfacial tensions between bitumen and aqueous phases were measured at process temperature (80 and 50 degreesC). The electrophoretic mobilities of the dispersed oil and fine mineral phases were also measured for each processibility determination. By varying the level of process aid addition, sub-optimal, optimal and overdosed conditions were represented. In the case of the oil/ solution interface, the present work shows that the interfacial tension is reduced and passes through a minimum with increasing natural surfactant-producing alkaline process aid addition. The condition under which this minimum is achieved is associated with maximum efficiency of the hot-water flotation process. In the case of the bitumen/aqueous interface, optimal primary recovery efficiency was found to be associated with a maximum in interfacial electric charge and a minimum in interfacial tension. (C) 2002 Elsevier Science B.V. All rights reserved.