A geochemical investigation has been carried out on the gas phase associated to thermal fluids discharged along three different segments of the East Anatolian Fault Zone (EAFZ, Turkey) running from Malatya to the Triple junction area (Karliova) where the East and North Anatolian Faults cross each other. CO2 is always the major gaseous component in both bubbling and dissolved gases with variable amounts of nitrogen helium and CH4. The isotopic ratios of helium range from 0.44 to 4.41Rac (values corrected for the atmospheric contamination) and cover a range spanning from crustal to magmatic-type values. The isotopic composition of carbon (CO2) shows values in the range from -5.6 to -0.2 parts per thousand vs PDB for the bubbling gases in contrast with the positive values (from 0.3 to 3.4 parts per thousand vs PDB) detected for the Total Dissolved Inorganic Carbon (TDIC). Coupling the information from the isotopic and chemical compositions, it results that mantle-derived fluids are driven to the surface by lithospheric structures. Despite the absence of outcropping volcanic products, the tectonic setting of the different segments plays a major role in releasing mantle-type fluids. The mantle derived fluids interact at shallower levels with circulating waters and originate geothermal systems which equilibration temperatures are estimated to be up to 360 degrees C The collected thermal fluids show different geochemical features consistent with processes occurring at two different levels: a deep level where mantle-originated fluids are taken either from the upper mantle or from intruded magma batches, and a shallower level, in the upper crust, where Gas Water Interactions (GWI), secondary CO2 production, and fractionation processes induced chemical and isotopic modifications of the pristine gas composition. (C) 2012 Elsevier B.V. All rights reserved.