Fluorinated SiO2 (SiOF) films, prepared by plasma enhanced chemical vapour deposition from SiH4, N2O and CF4 precursors, have been analysed by infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to extract chemical and structural information. Notwithstanding XPS reveals that fluorine concentrations are quite low (less than 4 at.%), the analysis of the Si-O-Si vibration modes in the LR spectra indicates that CF4 addition involves a deeper modification of the film structure, than the simple formation of Si-F bonds. In particular. by increasing the F concentration in the oxides, the stretching frequency of the Si-O-Si bonds increases. while the bending frequency decreases. On the basis of the central force model, both observations are consistent with the occurrence of a Si-O-Si bond angle relaxation phenomenon, the importance of which increases with the fluorine concentration in the films. (C) 2000 Elsevier Science B.V. All rights reserved.