In the period 1990–1995, 43 samples of dissolved gases in groundwaters (wells, springs and drainage galleries) and 64 samples of free gases (fumaroles, bubbling and soil gases) from the Mount Etna area were collected and analysed. Isotopic analyses were also carried out of both the carbon of the CO2 in free gases and dissolved inorganic carbon (DIC) in the waters. The chemical composition of the gases depends on the relative abundances of three interacting components. These are gases of (1) atmospheric origin enriched in O2 and N2, present almost exclusively in dissolved gases, (2) deep origin enriched in CO2, prevalent in the majority of cases, and (3) more superficial origin enriched in CH4, prevailing in peripheral areas. The isotopic composition of CO2 carbon confirms its mantle origin with either thermometamorphic CO2 contributions or modifications due to interaction with hot waters.