Pre-eruptive conditions and degassing processes of the AD 79 plinian eruption of Mt. Vesuvius are constrained by systematic F and Cl measurements in melt inclusions and matrix glass of pumice clasts from a complete sequence of the pumice-fallout deposits. The entire 'white pumice' (WP) magma and the upper part of the 'grey pumice' (GP) magma were saturated relative to sub-critical fluids (a Cl-rich H2O vapour phase and a brine), with a Cl melt content buffered at similar to 5300 ppm, and a mean H2O content of similar to 5%. The majority of the GP magma was not fluid-saturated. From these results it can be estimated that the WP magma chamber had a low vertical extent (<500 m) and was located at a depth of similar to 7.5 km while the GP magma reservoir was located just beneath the WP one, but its vertical extent cannot be constrained. This is approximately two times deeper than previous estimates. H2O degassing during the WP eruption followed a typical closed-system evolution, whereas GP clasts followed a more complex degassing path. Contrary to H2O, Cl was not efficiently degassed during the plinian phase of the eruption. This study shows that F and Cl behave as incompatible elements in fluid-undersaturated phonolitic melts. H2O saturation is necessary for a significant partitioning of Cl into the fluid phase. However, Cl cannot be extracted in significant quantity from phonolitic melts during rapid H2O degassing, e.g. during plinian eruptions, due to kinetics effects. Halogen contents are better preserved in volcanic glass (melt inclusions or matrix glass) than H2O, therefore the combined analysis of both volatile species is required for reliable determination of pre-eruptive conditions and syn-eruptive degassing processes in magmas stored at shallow depths. (C) 2008 Elsevier B.V. All rights reserved.