Fluorine and chlorine diffusion were measured in two natural phonolitic melts, from Vesuvius (Italy) and from Laacher See (Germany), at 0.5 and 1.0 GPa, between 1250 and 1450 degrees C at anhydrous conditions and with about 2 and 5 wt.% of dissolved water. The two different starting materials allow us to investigate the alkali effect, Na vs. K, on halogen diffusion. One composition was a K-rich (similar to 10 wt.%) phonolitic melt corresponding to the white pumice phase of the 79 AD eruption of Vesuvius, and the other a Na-rich (similar to 10 wt.%) phonolitic melt corresponding to most differentiated melt of the 12,000 BC eruption of Laacher See. The diffusion-couple technique in a piston cylinder was used for the experiments. Experiments were performed with only one halogen diffusing and with the simultaneous diffusion of a halogen mixture (F, Cl, Br) in order to evaluate the interactions between the halogens during diffusion. Diffusion coefficients for Frange between 2 x 10(-11) m(2)/s at 1250 degrees C and 7 x 10-(11) m(2)/sat 1450 degrees C for the Na-rich melt and between 1 x 10(-11) m(2)/s at 1250 degrees C and 8 x 10(-11) m(2)/s at 1450 degrees C for the K-rich melt at anhydrous conditions. Diffusion coefficients for Cl range between 2 x 10(-12) m(2)/s at 1250 degrees C and 1 x 10(-11) m(2)/sat 1450 degrees C for the Na-rich melt and between 7 x 10(-12) m(2)/s at 1250 degrees C and 2 x 10(-11) m(2)/s at 1450 degrees C for the K-rich melt at anhydrous conditions. Fluorine diffusivity is higher than Cl in the Na-rich-phonolitic melt by one order of magnitude, whereas in the K-rich-phonolitic melt F and Cl diffusivities are similar. The effect of water is significant for Cl in both Na-rich and K-rich melts: the addition of water enhances Cl diffusivity by up to one order of magnitude, but water does not significantly affect F diffusion. F and Cl diffusivities always differ from one another in the same phonolitic melt composition. F diffusivities are similar in both compositions. Conversely, Cl diffusion depends upon the dominant alkali. These results evidence that halogen diffusivity may represent a limiting factor for their degassing during rapid syneruptive decompression and vesiculation of H2O-rich-phonolitic melts. The contrasting volatile diffusivities of F and Cl in silicate melts during magma vesiculation may be a key, controlling factor of the composition of the vapour phase (bubbles) produced. Such diffusion controlled degassing model may explain the absence of F and Cl degassing observed during the 79AD eruption of Vesuvius. (C) 2008 Elsevier B.V. All rights reserved.