This work addresses the optical properties and chemical composition of carbon-containing silicon dioxide-like (SiCxHyOz) films obtained from oxygen/hexamethyldisiloxane mixtures in an argon-fed expanding thermal plasma at different O2 flow rates. The films, tested as potential low dielectric constant materials (low-k materials, k ˜ 2.8 at 1 MHz), have been extensively characterized in order to gain insight into their structure and chemical stability at the ambient air and under annealing conditions. The optical properties have been investigated by means of UV–VIS–NIR variable angle spectroscopic ellipsometry (VASE) and by optical transmission measurements. Rutherford backscattering and elastic recoil detection analyses provided the film density and chemical composition. IR-VASE was used to obtain information on the film chemical environment. These complementary techniques have suggested that the film characteristics range gradually from high refractive index, polymer-like SiCxHyOz films (n ˜ 1.64, ¿f ˜ 1.36 g/cm3) to silicon dioxide-like films (n ˜ 1.43, ¿f ˜ 1.72 g/cm3), as a function of O2 flow rate. Films deposited at low O2 flow rates proved to be porous and chemically unstable, as their exposure to ambient air led to water absorption and oxidation. On the contrary, films deposited at higher O2 flow rates, though still porous, are chemically stable and experience a limited water up-take. IR-VASE measurements at different annealing temperatures up to 300 °C have shown no decrease in carbon content, thus confirming the stability of these films under annealing conditions, an essential requirement for reliable dielectric materials.