Lee showed that molecular transport in all isothermal membrane processes, based on dense layers, can be described with one single general equation, including only measurable conditions from outside of the membrane. Following arguments given by Paul2 for homogeneous rubber membranes, Lee assumed the pressure everywhere inside the membrane to be equal to the feed side pressure. In this paper the question is stated whether this assumption always holds for very thin and/or nonhomogeneous membranes like asymmetric membranes made of semicrystalline polymers. A study based on less stringent assumptions of the pressure profile inside the membrane is performed. The results show that in most cases this pressure profile does not play a major role in the permeation process.