Hamamayagi thermal spring (HTS) is located along the North Anatolian Fault Zone. The thermal spring has a temperature of 36A degrees C, with total dissolved solids ranging from 485.6 to 508.5 mg/L. Hard, brittle, and gray limestones Permian aged are the reservoir rocks of the HTS. delta O-18-delta H-2 isotope ratios clearly indicate a meteoric origin for the waters. The delta S-34 value of sulfate in the thermal water is nearly 4.1aEuro degrees and implies a diagenetic environment characterized by reduced sulfur compounds. The delta C-13 ratio for dissolved inorganic carbonate in the HTS lies between -1.78 and -1.62aEuro degrees, showing that it originates from the dissolution of fresh-water carbonates. Quartz geothermometry suggests a reservoir temperature of 52-85A degrees C for the Hamamayagi geothermal field, but chalcedony geothermometers suggest reservoir temperatures between 30 and 53A degrees C.