The roles in the hot water process of sodium hydroxide, surfactants, water soluble electrolytes, fine solids and electric properties of the bitumen/water and solids/water interfaces are examined. Recovery of bitumen from low- to medium-grade oil sands can be increased by ensuring the fine solids remain dispersed during digestion by adjustment of digestion water salinity and pH. Recovery of bitumen from high-grade oil sands appears to be controlled by surfactants which affect bitumen water interfacial tension and the size of bitumen droplets formed during the initial separation of the bitumen from sand grains. Processing behaviour of mixtures of oil sands is explained within the framework of these mechanisms. Introduction In the past few years, there has been a renewed effort to understand the hot water process which is used by Suncor Inc. and Syncrude Canada Ltd. to commercially extract bitumen from mined Athabasca oil sand. The process can be considered in terms of two distinct stages; separation of the bitumen from the sand followed by flotation which encompasses both primary flotation and scavenging. Separation is induced by mixing the oil sand in a conditioning drum with hot water and often a process aid, usually sodium hydroxide. The conditioned slurry is pumped to a primary separation vessel (PSV) where the bitumen froth is skimmed from the surface. The sand tailings which settle in the PSV are removed. The middlings (water) are pumped to a secondary recovery vessel where air is introduced through the bottom of the vessel, attaches to some of the suspended bitumen and causes it to float. The amount of sodium hydroxide added to the conditioning stage of the process is the major variable used to maximize recovery of bitumen. The addition rate historically has been based on empirical but sometimes imprecise correlations of oil sand grade with processability. Recently Schramm and co-workers proposed that the process could be optimized around the concentration of free surfactants formed due to reactions between sodium hydroxide and organic acids in the bitumen, and found in the tailings water(l). Even if the surfactant concentration in the tailings stream of a hot water processing plant could be accurately monitored there would be a delay between the time recovery fell below its optimum level and an adjustment to the digestion stage of the process could be made. A model that is used to predict the best operating conditions based upon properties of oil sand that are easily measured either during a coring program or on-line, would be beneficial compared to one which is used in a reactive mode. This paper describes a model for the hot water process based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) and Ionizable Surface Group theories that can be used to predict from concentrations of inorganic salts in the digestion stage of the process, the conditions for maximizing recovery of bitumen from low grades of oil sand. The limitations of this approach especially for oil sands containing low levels of fine solids, arc addressed through consideration of the role of surfactants in the process.