Membrane technology is used for a wide range of separations from particle-liquid separations to gaseous and liquid-liquid separations. In this paper, we introduce a detailed model that describes a general membrane separation. The model disregards many common assumptions such as plug flow; constant temperature; constant pressure; binary mixture; steady-state conditions; and constant physical properties. Bur approach is applicable to any membrane separation and in this paper we demonstrate its application to bath liquid mixture separation (pervaporation) and gas separation in hollow-fibre modules. Both cases are seen to exemplify the need for a detailed model.