Films synthesized by plasma enhanced chemical vapor deposition from a mixture of octamethyltrisiloxane and hexamethylcyclotrisiloxane have been studied regarding to their preparation, deposition, chemical composition and membrane properties according to hydrocarbon vapor selectivities of solubility. Composition of the plasma glow discharge in neutral species has been studied by mass spectrometry whereas structural information of the deposited membranes has been extracted from Fourier transform infra-red (FTIR) spectroscopy. In the deposition conditions presented here leading to plasma-polymerized films, heavy radicals mostly contribute to their growth and their chemical composition. Depending on the precursors ratio in the plasma, i.e. linear and cyclic clusters ratio in the deposited material, solubility of selectivity against nitrogen of the deposited material varies from 50 up to 150 for hexane vapor. (C) 2003 Elsevier B.V. All rights reserved.