The oxygen permselectivity of a poly[1-(trimethylsilyl)-1-propyne) (PTMSP) membrane was drastically improved by plasma polymerization of fluorine-containing monomers. The effects of such plasma polymerization conditions as deposition time, plasma power and the type of monomer on the gas permeabilities of PTMSP membranes were studied. The best result of 1.41 x 10(-7) cm3 (STP) cm/cm2 s cmHg of oxygen permeability with a separation factor of 4.73 for oxygen/nitrogen permselectivity was obtained by plasma polymerization of trifluoromethylbenzene. The effect of substrates on the permeability characteristics of a plasma deposited layer was also evaluated. The results indicated that the permeability characteristics of plasma polymer depositions was related to the properties of the substrate.