Thin him membranes were polymerized from silicon organic monomers upon inorganic porous substrates in an RF plasma deposition process. The permeation properties of the films with a typical thickness of about 200 nm were investigated by the pressure increase method at temperatures up to 350 degrees C using He, N-2, CO2, CH4, C3H8 and C4H10. Three different types of membranes are presented exhibiting pure Knudsen-selectivity, solubility controlled permeation even at 350 degrees C, and molecular sieve effects. Their maximum operating temperature is limited to 425 degrees C since higher temperatures irreversibly change the plasma polymer morphology. The deposited membranes are well-suited to enhance the selectivity of thin film semiconductor gas sensors with which they are married to membrane/sensor microsystems.