The chemical composition and the reactivity of films deposited in RF glow discharges fed with Ar-tetramethylsilane and Ar-tetramethylsilane-O2 mixtures have been studied by X-ray photoelectron spectroscopy. The analyses have been performed by means of a spectrometer connected in vacuo to the deposition reactor. This configuration allows to analyze as deposited films without any external contamination of oxygen and moisture. The work has been conducted by studying the detailed Si2p, C1s, and O1s signals at different electron take-off angles, as well as the valence band region. Depth profiling of films by means of Ar+ ion sputtering has also been performed in some cases.