The present work deals with the quantitative mass transport of gases (N-2, He, O-2, CO2, H-2, CH4 and CO) through Langmuir-Blodgett (LB) systems deposited onto thin pore-free polydimethylsiloxane (PDMS) membranes as support, forming a composite system. For gas permeation experiments the number of LB layers of this composite system was systematically varied. The polyglutamate LB films OM30 and OM10 yield a perfect permeation behaviour. This can be proved by applying transport equations, properly taking into account the state and number of layers necessary for flux limitation by the deposited films. In this case permeation coefficients and selectivities can be calculated. The remarkable high gas selectivities of OM10/OM30 films (He/N-2, 22.7/8.3; O-2/N-2, 4.8/3.0; CO2/N-2, 43.5/27.8; H-2/N-2, 23.5/12.3; CH4/N-2, 1.8/2.6; CO/N-2, 1.3/1.6) are a consequence of the homogeneous and pore-free state of both polyglutamate films. The difference in permeation coefficient and selectivity of OM30 and OM10 is related to their difference in film density which is indicated by different film thicknesses as measured by ellipsometry. In contrast to polyglutamate layers, Cd-arachidate LB films perform as porous systems with a complete lack of selectivity. The microscopic imaging technique PSIM (phase-shift-interference microscopy) indicates a 3D-crystallisation (Ostwald-ripening) for Cd-arachidate LB films which are several days old. For the case of polyglutamate layers the reported gas selectivities are very promising, especially from a technological point of view.