The bulk diffusivity of dissolved CO 2 in obsidian and dacite melts containing 0–11wt% dissolved water at 800–1100°C and 1 GPa was characterized using a 14C radiotracer technique. Five values were also obtained for Cl in melts containing 8% H 2O. Carbon dioxide diffusivity ( D CO 2 ) in nominally dry molten obsidian is similar to previously reported values for a simple melt in the Na 2O-Al 2O 3-SiO 2 system and for dry basalt, indicating little effect of melt composition on bulk CO 2 diffusion. Three D CO 2 values for hydrous dacite melt are consistent with this lack of a melt composition effect. Dissolved water, however, causes an increase of D CO 2 in molten obsidian by about an order of magnitude for each 5% of added water, such that at 8 wt% H 2O D CO 2 = (6.5 × 10 −4) exp( −18000 ± 5000 RT ) where D CO 2 is in cm 2/s and R in cal/deg-mol. The activation energy of ~ 18 kcal/mol is lower by a factor of ~3 than that for diffusion in dry basalt. In comparison with existing data for diffusion of water in silicic melts at their typical liquidus temperatures, D CO 2 is generally lower unless the melt contains more than about 6% dissolved H 2O, in which case D CO 2 ~ D H 2O . The few data obtained for chlorine diffusion reveal that D Cl is lower by a factor of 5–10 than D CO 2 in the same melt. The overall systematics of the new data on dissolved volatile diffusion indicate that diffusional fractionation effects are likely to be most significant in silicic to intermediate magmas having water contents in the 0–4% range.