In many production processes it is impossible to prevent the occurrence of flue gas streams containing solvents even if production-oriented environmental protection measures are implemented consistently. This means that there is still an urgent need to minimize emissions by cleaning solvent-loaded process and flue gas streams. The thermal, catalytic or biological oxidation procedures that destroy organic solvents should only be used in cases where recovery is not reasonably possible. The procedures of choice are those that enable the solvent to be recovered and recycled, In the last ten years, major progress in the field of membrane separation technology has made gas permeation an interesting alternative to the conventional methods. The combination of a membrane separation step with direct or indirect condensation results in a hybrid process permitting gentle and contamination-free recovery of the organic components. The following article is a brief summary of the fundamental principles of the permeation of gas/vapor mixtures through polymer membranes.