The study area of Seferihisar is located within the Izmir-Ankara suture in the NW of the Menderes Massif in western Anatolia, Turkey. The Paleozoic metamorphic rocks of the Menderes Massif form the basement rocks in the area which are overlain by 760 m thick Izmir-flysch series consisting of metamorphic rocks, limestones and ultrabasic rocks tectonically. The Pliocene Bahcecik formation which consists of an alternation of conglomerates, sandstones, claystones, lignite and limestones and shows a thickness of 300 m overlies the (Izmir) flysch series discordantly. This is overlain by 430 m thick Yenikoy formation composed of conglomerates, sandstones, claystones and clayey limestones discordantly. The Miocene volcanic rocks of Cumaovasi overlie the Yenikoy formation concordantly which are overlain by alluvium and travertine deposits. Geothermal waters which are observed in the localities of Tuzla, Cumali, Doganbey and Karakoc are associated with NE-SW trending faults in the area. The geothermal waters in the area are considered as Na-Cl or Na-Cl-HCO3 type waters. The geothermal waters of Seferihisar and environs are identified to be Na+K > Ca > Mg dominant cations and Cl > HCO3 > SO4 dominant anions. According to the diagram of Na/100-K/100-root Mg, a certainly part of the thermal waters can be considered as equilibrated thermal waters during some waters are of immature waters. According to the results of geochemical thermometers, the reservoir temperatures of thermal waters range from 150 to 240 degrees C. The delta H-2 values of thermal waters are between -13,3 to -31,9, while delta O-18 values range from -2,55 to -5,70. The tritium contents of thermal waters are between 13 to 64 +/- 10 TU.