Plasma enhanced chemical vapour deposition of SiO2 and polymeric SiO3C3H5 thin films has been carried out at room temperature in a microwave electron cyclotron resonance (ECR) reactor. Si(CH,),Cl has been used as volatile precursor of Si and pure oxygen as plasma gas. Plasma conditions were characterised by optical emission spectroscopy as a function of the relative flow rates of oxygen and precursor. The oxygen plasma was characterised by emission lines and bands due to O* and O-2(-) species whose relative intensity decreased as the flow rate of the precursor increased. Then, the plasma was dominated by the emission lines of H* species formed by dissociation of the precursor molecules, From the evolution of the intensity of the emission of oxygen and hydrogen lines as a function of the relative concentration of oxygen and precursor and by considering the composition and microstructure of the obtained thin films. a model is proposed for the decomposition mechanism of the precursor. According to this model. Si-Cl bond would dissociate in a first step. Then a series of reactions would follow with the activated oxygen species that, depending on the relative flow rate of oxygen, lead to the formation of SiO2 or a polymeric SiO3C4H2 material. The chemical composition of the films was analysed by Rutherford backscattering spectroscopy (RBS), electron recoil detection analysis (ERDA) and X-ray photoelectron spectroscopy (XPS), while their structure and microstructure were investigated by means of transmission electron microscopy (TEM), atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FF-IR). It has been also shown that the SiO,C,H. thin films, with typical compositions such as Si:1, O:2, C:3.6, H:5.5, yield SiO2 thin films by exposure to a plasma of oxygen. These SiO2 thin films were smoother than the parent SiO3C4H7 samples and the silica films prepared by PECVD under oxygen rich conditions.