There is an urgent need to better constrain the global rates of mercury degassing from natural sources, including active volcanoes. Hitherto, estimates of volcanic fluxes have been limited by the poorly-determined speciation of Hg in volcanic emissions. Here, we present a systematic characterisation of mercury partitioning between gaseous (Hg(g)) and particulate (Hg(p)) forms in the volcanic plume of Mount Etna, the largest open-vent passively degassing volcano on Earth. We demonstrate that mercury transport is predominantly in the gas-phase, with a mean Hg(p)/Hg(g) ratio of ∼0.01 by mass. We also present the first simultaneous measurement of divalent gaseous mercury (HgII(g)) and total gaseous mercury (Hg(g)) in a volcanic plume, which suggests that Hg0(g) is the prevalent form of mercury in this context. These data are supported by the results of model simulations, carried out with HSC thermodynamic software. Based on a mean ‘bulk plume’ Hg/SO2 mass ratio of 8.7×10-6, and a contemporaneous volcanic SO2 flux of 0.8 Mt·yr-1, we estimate an Hg emission rate from Mt. Etna during passive degassing of 5.4 t·y-1 (range, 1.1-10 t·y-1). This corresponds to ~0.6% of global volcanic Hg emissions, and about 5% of Hg released from industrial activities in the Mediterranean area. published 7377-7388 1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive 4.5. Degassamento naturale JCR Journal