Fluorine diffusion has been studied between 1600 and 2000 degrees C in vapour-deposited silica layers. In particular, the influence of the common impurities hydroxyl and chlorine and the codopant phosphorous oxide was investigated. Hydroxyl and especially phosphorous enhance the fluorine diffusion rate whereas chlorine shows no detectable effect. The determined Arrhenius dependence of the diffusion coefficient agrees well with previous investigations made at lower temperatures. Additionally, diffusion coefficients for chlorine were derived.