The viscous behavior of fluorine-doped synthetic silica is studied using collapsing experiments with different fluorine-doped tubes on a modified chemical vapor deposition(MCVD) lathe. The principles, techniques, and evaluations of this method are the same as the ones demonstrated previously in detail with pure and doped silica [1]. The present investigations provide information about the influence of fluorine doping up to a concentration of about 10 mol% F (3.4 wt% F) in a temperature range between 1600°C and2000°C. Fluorine doping leads to a systematic decrease in the viscosity, combined with a decrease of the activation energy of the viscous flow and a certain increase of the pre exponential factor. In summary, this demonstrates the weakening influence of fluorine on the glass network, similar to the incorporation of hydroxyl or chlorine.