The Sfax Basin in eastern Tunisia is bounded to the east by the Mediterranean Sea. Thermal waters of the Sfax area have measured temperatures of 23-36A degrees C, and electrical conductivities of 3,200 and 14,980 mu S/cm. Most of the thermal waters are characterized as Na-Cl type although there are a few Na-SO4-Cl waters. They issue from Miocene units which are made up sands and sandstones interbedded with clay. The Quaternary sediments cap the system. The heat source is high geothermal gradient which are determined downhole temperature measurements caused by graben tectonics of the area. The results of mineral equilibrium modeling indicate that the thermal waters of the Sfax Basin are undersaturated with respect to gypsum, anhydrite and fluorite, oversaturated with respect to kaolinite, dolomite, calcite, microcline, quartz, chalcedony, and muscovite. Assessments from various chemical geothermometers, Na-K-Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature of the Sfax area can reach up to 120A degrees C. According to delta O-18 and delta H-2 values, all thermal and cold groundwater is of meteoric origin.