The behavior of refractive index as a function of fluorine content of SiOF films deposited by remote plasma-enhanced chemical vapor deposition (RPECVD) from SiF4/H-2/O-2/Ar mixtures was studied. The refractive index and fluorine content of the films was evaluated by ellipsometry, infrared spectroscopy and resonant nuclear reactions (RNRs). The contributions of the electronic polarizability and porosity to the experimentally found reduction of the refractive index of the films, as a function of fluorine content, were calculated using the Clausius-Mossotti relation and the Maxwell-Gamett equation of the effective medium theory, in combination with a pseudo-binary alloy representation [(SiO2)(x)(Si2O3F2)(1-x)] proposed recently for the SiOF system. We found that the reduction in the electronic polarizability of this alloy due to the incorporation of fluorine has a minimal contribution to the reduction of the refractive index of the SiOF films. Therefore, the main mechanism responsible for this reduction is the porosity generated in the films as fluorine is incorporated in its network. (C) 2004 Elsevier B.V. All rights reserved.