and IntroductionSteam Assisted Gravity Drainage (SAGD) is a steam-flood process that relies on thermal diffusion and gravity to mobilise bitumen in oil sands reservoirs and allow for recovery. A pair of parallel horizontal wells are drilled near the bottom of the pay zone with approximately 5m vertical separation between them. After an initial circulation phase to establish communication, high quality steam is continuously injected down the injection well while the heated bitumen and condensate flow, due to gravity, to the production well. The volume of low oil saturation left behind is filled with steam and referred to as the steam chamber. The steam chamber grows vertically until it hits the cap-rock at which point it grows laterally away from the horizontal wells (1) .A major cost of producing bitumen by SAGD is the energy required to produce steam; thus there is a great drive to improve the energy efficiency of a SAGD project. Solvent addition is expected to increase oil production and improve the energy efficiency of SAGD by combining the thermal process with the diluent mechanism of a solvent (2) . A solvent is added to the steam chamber by injecting the solvent-steam mixture continuously from the injector well (3) .Reservoir simulations of solvent-steam co-injection were done with a pseudo-compositional thermal reservoir simulator. The effects of co-injecting a multi-component solvent, naphtha, and a single component solvent, propane or pentane, were compared to SAGD. Three pseudo-components were used to model the multi-component behaviour of naphtha.Co-injection of any of the solvents studied (propane, pentane, naphtha) resulted in an improved SOR. The greater the mole ratio of solvent to steam the greater the improvement in SOR. Only naphtha co-injection resulted in an improved oil production rate. All components of naphtha travelled freely in the vapour chamber and accumulated along the vapour chamber front in both the vapour and oil phases. At higher solvent to steam ratios oil production rate decreased due to an accumulation of solvent gas at the vapour chamber front.