The hexamethyldisiloxane/oxygen-fed expanding thermal plasma deposition technique was used for a step-by-step tailoring of the film chemistry from C-rich polymeric films (so called silicone-like, SiCxHyOz) to SiO2-like films. Information related to the chemical composition of the film was obtained by means of infrared spectroscopy. In situ ellipsometry (632.8 nm) monitored the film growth in real time, providing the deposition rate and refractive index. On the basis of ex situ UV-Vis spectroscopic ellipsometry, an optical model is proposed in order to determine film porosity and to explain the presence of a small absorption in SiO2-like films, attributed to residual carbon presence.