The vapour-liquid phase behaviour of steam-solvent (light hydrocarbon) systems used in an oil sands recovery process called steam assisted gravity drainage (SAGD) is examined. Analysis shows that condensation occurs over a temperature range and that a concentration gradient exists between the liquid and vapour phases. In a large range of solvent concentrations, water condenses first from the vapour phase. Solvent only condenses first from the vapour phase at extremely high solvent volume fractions. Due to water condensing first at most concentrations, the ability of the solvent to directly contact the bitumen in the reservoir depends on the orientation of the vapour-liquid interface and the relative position in the vapour chamber. Addition of solvent into a mature SAGD operation can also cause a temporary suppression of the steam-oil ratio (SOR) due to the change in the temperature at the vapour-liquid interface. This effect must be taken into account in interpreting experimental or simulation results. The addition of solvent not only changes the temperature but also decreases the heat of condensation of the mixture. As the concentration of solvent changes, the SOR is also expected to change. (C) 2011 Elsevier Ltd. All rights reserved.