Separation of a mixture of toluene and heptane by means of a liquid surfactant membrane was studied. With the help of a membrane film model for mass transfer through a liquid membrane, the validity of the assumption made by Casamatta et al, in their study of hydrocarbon separation through a liquid water membrane was determined. This paper also proposes a never method to determine the exact thickness of the liquid membrane through which permeation takes place, and thus eliminates any possibility of underprediction of the extraction rate as is made by other investigators considering the maximum membrane thickness.