Silicon oxides ranging from near stoichiometric to silicon rich silicon oxides can be obtained by plasma enhanced chemical vapour deposition from silane (SiH4) and nitrous oxide (N2O) mixtures. During deposition, impurity bonds like Si–H and Si–OH incorporate into the oxide matrix depending on the deposition parameters and mainly on the precursor gas flow ratio, R = QN2O / QSiH4. In this work, plasma deposition from SiH4 and N2O has been modelled, in order to discuss the possible pathways that lead to the thin film composition variations with reactants flow ratio. It has been found that the SiH2/SiH3 ratio determines whether the film deposition takes place through silanol precursors (SiH3O, SiH2O…), leading to the incorporation of Si–OH bonds into the oxide film, or it is done through SiO, Si and silane hydrides, with the result of oxygen vacancies formation in the films and the introduction of Si–H bonds.