Thermal heavy oil recovery processes use steam and therefore are energy intensive and emit significant amounts of greenhouse gases. One option to reduce steam usage is a solvent-assisted process where a hydrocarbon solvent is co-injected with steam. A potential issue for solvent-assisted processes is the effect of the condensing steam on the mass transfer rate of solvent into the heavy oil. This thesis focuses on the diffusion of solvent into the bitumen when a layer of condensed water is present. The diffusion experiments were performed in 2.2 cm diameter glass vials. Water layers with thicknesses from 0.25 to 2 cm were placed over bitumen and excess solvent was placed over the water. The diffusion rate of the solvent through the water was determined from the change in height of the bitumen layer. A cathetometer was used to measure the changes in water and bitumen heights. The diffusion of toluene, cyclohexane, n-pentane and n-heptane into two bitumens from different source reservoirs was evaluated at temperatures from 20 to 80°C. Solvent diffusion rates were calculated from the change in the height of the water+bitumen layers and the emulsification rates were determined from the additional change in height of the bitumen layer. The water content in the bitumen was verified by Karl Fischer titrations and micrography. The solvent diffusion rates were modeled with a 1D steady-state diffusion model using diffusivity and solubility values for solvent in water taken from the literature. It was found that surfactants naturally present in bitumen were responsible for water-in-oil and oil-in-water spontaneous emulsification. At 20°C, the measured diffusion rates were consistent with the predicted diffusion rates and the water was a significant barrier to mass transfer. At 60°C, the diffusion rates were significantly higher than predicted possibly due to convection induced from small thermal gradients in the temperature bath but also due to gravity instability. The rate of emulsification rates and time to gravity instability correlated to the solvent mass transfer rates. It is expected that at steam temperatures the water will pose little barrier to mass transfer because gravity instability, convection, and emulsification will dominate.