In the two areas studied (Iran and the south of France), the thermomineral springs and the CO 2 emissions are always limited to great recently-active tensional fractures and sometimes accompanied by volcanic phenomena. In eastern Azerbaijan, Iran, springs are located on a branch of the large Tabriz fault, which intersects the Quaternary Sabalan volcanic massif at its periphery. In the French Massif Central and the western Alps, they are located on secondary fractures belonging to two families of faults directed N50°E and N140°E. The meteoric origin of the water is proved by the presence of tritium. Very different contents in the same area indicate these springs to be mixtures with variable proportions of deep, ancient hot waters and more recent subsuperficial waters. The 18O and 2H contents of the Azerbaijan springs are very homogeneous and indicate recharge by rain water, infiltrating the Sabalan volcanics. The enrichment in 18O by exchange with the wall rocks of four springs corresponds to the highest temperature of the reservoir given by the geothermometers, which is more than 300°C. This exchange is a determining factor in the geothermal research. On the other hand, the representative points of the springs of the southeast of France on a 18O— 2H diagram are all located on the meteoric water line according to the very different altitudes of their recharge zones. The major part of the 13C contents of the free CO 2 sampled at the springs are included in the range of −4 to −8‰ vs. PDB, which characterizes a deep CO 2 from the upper mantle. The higher 13C-values of the Bouchli gas in Iran (−2‰) and Kizildere in Turkey (−0.9‰) show that the CO 2 originates partly, or wholly, from thermal decomposition of the carbonate rocks. The points where an 18O enrichment of the water is observed are also the hottest points. The 13C depletion of the CO 2 in some mineral springs of southeast France down to values of −14‰ results from an isotopic exchange of the magmatic CO 2 with the organic carbon of the coal beds — which act as a temporary storage formation — during the transit of the gas towards the surface.