The aim of this article is to show the importance of concentration polarization effects in the separation of gas-vapor mixtures using membranes. In the experimental part of this work, gas mixture measurements are conducted with a specially designed test cell. The experimental data are analyzed using a two-resistance model for the transport through the membrane, which is derived in the theoretical part of this work. The two resistances considered are the transport through the boundary layer on the feed side of the membrane and through the separation layer. For the transport through the separation layer an extended free volume model is derived. This model considers not only the feed side but also the influence of the permeate side on the separation properties of the membrane. The results of the measurements show the influence of concentration polarization effects and their dependencies on feed pressure, membrane thickness, and feed flow rate.