Аннотация:Essentuki mineral groundwater (EMGW) basin is characterized by unique and complex geological and hydrogeological settings due to monoclinal character of the geological structure, breaking by north-eastern faults, significant stratum of clays, overlaying major aquifers, volcanic laccolite peaks. EMGW area involves the unique wide variety of mineral waters with different TDS, pH, chemical, and gas composition. TDS changes from fresh (0.5–0.9 g/L) to high salinity (10–13 g/L) and strongly depends on the chemical composition of water. The region is characterized by a wide spread of high pCO2 sparkling Ca–Na–HCO3–Cl waters, which are the trademark of “Essentuki 4” and “Essentuki 17”. Such waters have strong therapeutic properties. Geothermal conditions, major- and micro-elements, isotopic characteristics of water (δ18O and δ2H) and gas (δ13C in CO2 and CH4, 3He/4He) phases of carbon dioxide mineral waters of EMGW basin were analyzed in this paper. The CO2free gas is probably a mixture of gas from deep (volcanic) and biogenic sources, although methane has the marine microbial origin and evolves from the degradation of dispersed organic matter in the host rocks. 3He/4He values prove that the free associated gas in mineral groundwaters of the region is a mixture of several sources of gas (mantle, crustal, and biogenic).