Polysiloxanes such as polydimethylsiloxane (PDMS) combine several unique properties. They exhibit a low glass transition temperature, high thermal and oxidative stability, high UV-resistance, low surface energy and strong hydrophobicity as well as high permeability to many gases. These characteristics are independent over a wide temperature range. Our research deals with the synthesis and properties of thermoplastic silicone elastomers containing soft segments of dimethylsiloxane and hard urea segments. In order to determine structure-property relations, regarding to permeation properties, the chemical structure was varied. In this study thermoplastic silicone elastomers, based on polydimethylsiloxane oligomers with amino groups and different diisocyanates, were synthesized. The polymers were characterized by nuclear magnetic resonance (NMR) and infrared (FTIR) spectroscopy. The formation of hydrogen bonding of the hard segments was investigated by temperature dependent FTIR-spectroscopy, which demonstrated the thermal stability of the elastomer and defined the processing conditions of such polymers. Thin membranes of thermoplastic main-chain urea-siloxane copolymer were prepared and investigated with respect to the permeation properties of organic solvents. Vials were filled with a mixture of an aromatic (toluene) and an aliphatic hydrocarbon (iso-octane) and were sealed with the polymer films. The concentration of residual solvent mixture was measured by the relation of the peak area of the two solvents. This was investigated by the GC-MS-technique. A separation of the solvent mixture occurred. In conclusion, novel main-chain urea-siloxane thermoplastic elastomers were successfully synthesized with selective barrier properties. These polymers are highly permeable for organic solvents depending on the chemical structure. These siloxane polymers can be used as semipermeable membranes for separation of solvent mixtures in oil field applications.