Mygdonia Basin is an active seismotectonic zone. Its regional geology is characterized by step-faulted metamorphosed Paleozoic and Mesozoic sediments partly intruded by igneous rocks. Two geothermal anomalies, heating groundwater to more than 40 degrees C at 100 m below the surface, are tied to horst-like basement uplifts along deep-reaching faults. Inorganic carbon dissolved in thermal waters of the region is isotopically rather heavy, indicating an endogenic origin. In spite of the considerable variation, delta(18) does not allow differentiation of thermal and non-thermal waters. It does not agree with the isotopic pattern of recently recharged groundwaters. delta(13)C in fossil spring deposits of tufa towers and bedded travertines indicates a derivation from endogenic CO2 similar to that dissolved in present thermal waters. In contrast, their delta(18)O is remarkably high, suggesting evaporated basin waters as a possible source.