Thin films were polymerized from different organosilicon compounds in a radio-frequency plasma deposition process. The properties of the layers were characterized with respect to the deposition rate, the density, the refractive index, and the chemical structure determined by FTIR and XPS analysis. The qualification of the films for gas-selective membranes was tested on different porous substrates using N-2, H-2, O-2, CO2, and CH4. Both structure and permeation performances of the synthesized films were correlated with the composite plasma parameter V/F . M (V: input voltage; F: monomer flow rare; M: monomer molecular weight). At low V/F . M ratio, the thin layers are mainly constituted of the [(CH3)(2) -Si(-O)(2)] environment (monomer and polydimethylsiloxane one). Increasing the V/F . M results in a more "inorganic" chemical structure, higher O/Si ratio, refractive index, and density, the materials tend toward a silicalike structure. Concurrently the prepared membranes have solution-diffusion-controlled or Knudsen-like separation factors, depending on whether plasma conditions are soft or hard.