Thin ppHMDSO plasma polymer films deposited in a low pressure hexamethyldisiloxane plasma have been exposed to oxygen plasmas ignited in the same reactor at different pressures. The time evolution of the ppHMDSO film during the oxygen plasma treatment (30 s to one hour) was monitored in situ bymulti-wavelength ellipsometry. The treated films were analysed ex situ by X-ray reflectometry, spectroscopic ellipsometry and infrared spectroscopy. On the other hand, the oxygen atom density aswell as the oxygen ion density and energy weremeasured by actinometry and Langmuir probe measurements, respectively. For all the investigated oxygen plasma conditions, the film thickness and extinction coefficient significantly decrease. If the O2þ ion energy is low enough (typically less than 10 eV) the 65nm ppHMDSO can even be transformed into a 45 nm-thick carbon free SiO2-like film. In contrast, if the ion energy is higher than 10 eV, the ppHMDSOis only partially oxidised. Based on the oxygen atom flux measurement, it is concluded that the number of oxygen atoms impinging on the ppHMDSO film is not the only parameter which governs the treatment. The oxygen ion energy appears to have a key role as far as energetic ions are likely to induce the formation of a dense superficial SiO2 layer which prevents the diffusion of O atoms in the film and the out diffusion of volatile oxidation products.