Fluorine is a common volatile component in silicate melts of all compositions. Although less abundant than water, fluorine is known to reduce significantly the liquidus temperature and the viscosity of granitic liquids, as does water. In contrast, previous studies suggest that fluorine has little or no effect on the viscosity of basaltic liquids. We have studied the effect of fluorine on dacitic and rhyodacitic liquids, to see if fluorine has a strong or weak effect on their viscosity; the results may be important for a better understanding of the rheology of subduction zone magmas. In our experiments, we synthesize iron-free dacitic liquids from oxide and carbonate powders, and incorporate fluorine by using varying amounts of calcium fluoride instead of calcium carbonate. The fluorine contents of the liquids are 0.03, 0.09, 0.30, 0.79 and 1.43 weight percent. After decarbonation, and repeated cycles of melting and grinding to ensure homogeneity, the samples are quenched to a glass. Cylindrical cores are drilled from the glass and the viscosity measured by parallel plate viscometry, over the range 10⁹ to 10 ^12.5 Pa.s. For the dacites, this corresponds to temperatures between about 700 and 840°C, i.e. relevant magmatic temperatures. At 800°C, the viscosity of fluorine-absent dacite is 6.2 Χ 10 ¹⁰ Pa.s. The addition of 0.03 wt.% and 1.43 wt.% fluorine decreases the viscosity by factors of about 3 and 250, to 2.3 Χ 10 ¹⁰ and 2.3 Χ 10 ⁸ Pa.s, respectively. This compares with a viscosity of about 1.1 Χ 10 ⁶ Pa.s for the same dacite containing 1 wt.% water. Even on a mole percent basis, water is far more efficient at reducing the viscosity of dacitic liquids than fluorine. This suggests that dissolution mechanisms other than simple depolymerization of the silicate network are operating. The effect of fluorine is less strong on dacites than on haplogranitic melts. Ongoing experiments will test the effect of fluorine on more polymerized rhyodacitic melts.