Thin SiOxCvHz films have been prepared by plasma enhanced chemical vapor deposition (PECVD) on silicon substrates at low pressure (2 mTorr) and 300 W rf power, using tetraethoxysilane (TEOS) or hexamethyldisiloxane (HMDSO) as a monomer and oxygen as a reactive gas. The plasma composition, the structure and properties of the deposited films are studied as a function of the organosilicon fraction (X-org). Optical emission spectroscopy is carried out in order to identify the species in the plasma. The layers are characterized by in situ spectroscopic ellipsometry and by several ex situ diagnostics including infra-red spectroscopy, X-ray photoelectron spectroscopy, gravimetry and chemical etching. At low values of X-org, the structure and properties of the films and optical emission spectra are very similar whatever the organosilicon precursor. At high values of X-org, the structure and properties of the deposited films and emitting species significantly depend on the organosilicon precursor.