industrial processes commonly emit organic components as dilute or concentrated streams. Environmental regulations impose stringent emission standards, interest in membrane separation techniques has grown along with their acceptance for vapour recovery. They compete with absorption, adsorption, cryogenics and incineration. In research carried out In collaboration with GKSS, Geesthacht, Germany, the separation of gases and vapours was experimentally investigated for various thin film, composite membranes in a flat sheet configuration, with a dense elastomeric silicone film layer over a porous PEI or PVDF support. Available literature data were reviewed and assessed including tbe models to describe mass transfer through the membranes. Experimental results for pure substances and for binary mixtures were used to determine the permeability as a function of temperature and to calculate the selectivity toward N-2. The free-volume model  approach was used to predict the permeabilities of hydrocarbons. The necessary surface area of the membrane separation module was determined for a given flow rate, concentration, operating temperature and pressure and from the characteristic selectivity of the membrane. The design is illustrated by the separation of butane from an air waste stream produced by polyethylene foaming and for the CH4-enrichment of a landfill biogas. A sensitivity analysis towards operating parameters illustrated the predicted effects. Finally, current industrial applications ave briefly reviewed, e.g. enriching landfill gas, recovery of VC-monomer from PVC-process off gas, etc.