Fluorinated silica has a dielectric constant E in the range of 3—3.5, lower than that of F-free SiO2 (ω=4). The reasons behind this reduction are controversial. It is not known whether the electronic or ionic contributions to the overall screening are being diminished upon F doping. To shed more light on this phenomenon we have studied F-doped SiO2 with ab-initio modeling and various characterization techniques. FTIR transmission and spectroscopic ellipsometry give us information about the ionic and electronic contributions to ω Nuclear reaction analysis and Auger spectrometry measure F composition. XPS and FTIR provide information on the atomic structure and stability of the film. We use a large cell of cristobalite to model fluorinated silica theoretically. The ground state geometry is obtained via energy minimization. We calculate the vibrational density of states and find a localized mode (Si-F stretch), in good agreement with FTIR transmission. We analyze the effects of F incorporation on the dielectric properties.