The use of steam-assisted gravity drainage (SAGD) to recover bitumen from Athabasca deposits in Alberta has been growing Butler [Butler, Can. Pet Tech 1985,24.42-51] derived a simple theory to calculate the production rate of oil during SAGD in an ideal reservoir This simple and useful theory made several assumptions about the properties of the reservoir and operating conditions of the process. The theory also assumed that the highest mobility oil is at the edge of the steam chamber and that the oil phase velocity is highest at the chamber edge and reduces with distance into the oil sand This research examines flow conditions at the edge of the steam chamber Specifically, a new theory is derived that takes into account the impact of oil saturation and relative permeability on the oil mobility profile at the edge of a steam chamber. It is shown that the flow behaviour at the edge of a steam chamber is more complex and is not fully represented by Butler's theory. Contrary to Butler's theory, the oil mobility has its maximum some distance away from the edge of the steam chamber The results reveal that the higher the thermal diffusivity of the oil sand, the deeper the location where the oil phase velocity is maximum. The developed model has been validated against published experimental and field data