Thin films were deposited onto porous substrates by plasma polymerization using three kinds of organosilicic compounds, tetramethylsilane (TMS), hexamethyldisiloxane (Mz), and octamethylcyclotetrasiloxane (D,). Those composite membranes showed different characteristics of gas permeability. When D, was plasmadeposited onto a porous substrate, the composite membrane showed the highest oxygen permeability and the lowest oxygen-to-nitrogen permeability ratio. The composite membrane prepared from TMS showed the permeability characteristics opposite to the membrane obtained from D,. Infrared spectrum of the polymer from D4 resembles that of dimethylpolysiloxane. The plasma polymers from TMS and Mz showed different profiles in Si-0 absorption bands in the range 1100-lo00 cm-1 or in absorption bands of SiCH3 groups in the range 850-750 cm-l from respective monomers. Xray photoelectron spectroscopic observation indicated that all the plasma polymers contained more than two species of Si atom with different oxidation states. The greater part of Si atoms in plasma polymers took the same oxidation states in corresponding monomer. The gas permeability characteristics were closely related to the oxidation states of Si atom in the plasma polymers.