The studied area is a 130 km long fast spreading graben in Central Greece. Its complex geodynamical setting includes both the presence of a subduction slab at depth responsible for the recent (Quaternary) volcanic activity in the area and the western termination of a tectonic lineament of regional importance (the North-Anatolian fault). A high geothermal gradient is made evident by the presence of many thermal springs with temperatures from 19 to 82 °C, that discharge along the normal faults bordering the graben.In the period 2004–2012, 58 gas and 69 water samples were collected and their chemical and isotopic analysis revealed a wide range of compositions.Two main groups of thermal waters can be distinguished on the basis of their chemical composition. The first, represented by dilute waters (E.C. <0.6 mS/cm) of the westernmost sites, is characterised by the presence of CH 4 -rich and mixed N 2 –CH 4 gases. The second displays higher salinities (E.C. from 12 to 56 mS/cm) due to mixing with a modified marine component. Reservoir temperatures of 150–160 °C were estimated with cationic geothermometers at the easternmost sites.Along the graben, from west to east, the gas composition changes from CH 4 - to CO 2 -dominated through mixed N 2 –CH 4 and N 2 –CO 2 compositions, while at the same time the He isotopic composition goes from typical crustal values (<0.1 R/R A ) up to 0.87 R/R A , showing in the easternmost sites a small (3–11%) but significant mantle input. The δ 13 C values of the CO 2 -rich samples suggest a mixed origin (mantle and marine carbonates).