Oxygen and water vapor barrier coatings were made by plasma enhanced chemical vapor deposition on a polymer multilayer substrate. Plasma deposition was realized on the external PET layer of the substrate by using a RF plasma source and two different precursors (trimethylsilane (TMS) and hexamethyldisiloxane (HMDSO)). Two types of monolayer deposits (SiOx and SiOxCzHw) were investigated for each precursor. The structure and composition of the deposited coatings were characterized by FTIR spectroscopy and XPS analysis. Their surface topography was imaged by AFM and the surface energy was determined thanks to contact angle measurements. Oxygen and water barrier properties of the coated substrate were interpreted in terms of coating structure and composition and TMS was identified as the most interesting precursor for enhanced barrier properties. The sensitivity of the coatings to thermal exposure was also investigated. A multilayer deposit approach consisting in alternating organic and inorganic deposited layers was developed. By this route, additional significant enhancement of oxygen and water barrier performances was achieved upon 3 layer deposition.