The aim of this work is to characterize and understand the effects of oxygen plasma post-treatments on plasma polymerized hexamethyldisiloxane (ppHMDSO) layers, as these post-treatments on plasma polymerized layers are an alternative way to produce SiOx (x approximate to 2) films. Oxygen discharges on ppHMDSO films in continuous plasma (DC) and microwave multipolar plasma excited by distributed electron cyclotron resonance plasma (MW) are compared by studying their effects on the post-treated layer thickness, chemical composition, film structure and functional properties. Fourier Transform Infra-Red and X-ray Photoelectron Spectroscopies have been used to determine the film structure and composition of the initial ppHMDSO films and the post-treated layers. The oxygen post-treatment etches the end bonds and leads to a cross-linking of the polymer chains (formation of SiOC bonds). Oxidation process is strongly linked to the plasma type. A pre-existing model for the oxide growth has been applied, showing that the MW process is more effective than the DC. The main conclusion is that the structure of the resulting films is different for each process and that the oxygen diffusion coefficient is linked to the film structure. Higher oxygen diffusion is achieved in MW films, which are less dense than the films obtained by using the DC technique. It is also shown that the film properties (such as wettability, hardness and electrical and optical properties) are modified after the oxygen post-treatment. Finally, we briefly discuss on the most appropriate process to obtain these films according to the final application of the coatings. (c) 2006 Elsevier B.V. All rights reserved.