A geochemical investigation was undertaken at Mt. Etna Volcano to better define groundwater characteristics of its aquifers. Results indicate that the Na-Mg ± Ca-HCO3 − ± (SO4 2− or Cl −) type accounts for more than 80% of the groundwater composition in the volcano. The remaining 20% is characterized by elevated Ca 2+. Waters along coastal areas are enriched in SO4 2− or Cl − , mainly due to mixing with seawater and anthropogenic effects. The majority of the samples showed values between −4‰ to −9‰ for δ 18 O and −19‰ to −53‰ for δ 2 H, suggesting that precipitation is the predominant source of recharge to the aquifers, especially in the west of the study area. The analysis of δ 13 C and pCO2 shows values 1 to 3 times higher than those expected for waters in equilibrium with the atmosphere, suggesting a partial gas contribution from deep sources. The diffusion of gasses is likely to be controlled by tectonic structures in the volcano. The ascent of deep brines is also reflected in the CO2 enrichment (up to 2.2 bars) and enriched δ 2 H/δ 18 O compositions observed in the salt mounts of Paternò.