More than 200 chemical and isotope analyses of fumarolic fluids collected at the Fossa Grande crater, Vulcano Island, during the 1980s show that the main process controlling these fluids is mixing between the gas released by a magma body and the vapour produced through evaporation of brines of marine origin. Large variations in the relative contribution of these two sources have been observed during the last 10 a. The main species (H2O and CO2), the inert gases (He and N2), and the D content of steam are fixed by the mixing processes; they are therefore the best tracers of the fraction of the deep magmatic component in the fumarolic fluids discharged at the surface. In contrast, the ''fast'' species (H-2 and CO) equilibrate at T,P values close to the outlet temperature and atmospheric pressure, and under redox conditions governed by the SO2-H2S buffer, as indicated by thermodynamic calculations. Acid gases (HCl, HF, H2S and SO2) are partly contributed by the magmatic component and partly produced by the reactions between hot rocks, steam and salts which take place in the ''dry'' zones surrounding the central magmatic gas column, as suggested by the good agreement between their analytical and theoretical contents.
