The geothermal fields of Cesme are located in the western part of province capital of Izmir and can be considered as important tourist resorts with a great number of thermal hot springs which are used for thermal bathing since several years. Nowadays, the geothermal waters in the fields are used for district heating and greenhouses. In the area of Cesme, there are sedimentary and volcanic rocks predominantly. The basement rocks are of Devonian age and consists of intercalations of sandstones, greywackes and limestones overline by Upper to Middle Triassic carbonate rocks with intercalations of sandstones and claystones. These rocks are overlain by Neogene volcanic and terrestrial sedimentary rocks. In 1995, we have measured in-situ parameters in many locations of groundwaters and geothermal waters with collection of a great number of samples for these waters. The geothermal waters are of Na-Cl, Na-(Cl)-HCO3, Na-Ca-Cl, Na-Mg-(Cl)-HCO3, Na-Mg-Ca-(Cl)-HCO3 and Ca-Na-Mg-(Cl)-HCO3 type waters during the groundwaters display Na-Cl, Na-HCO3, Na-Mg-(Cl)-HCO3, Na-Mg-HCO3, Na-Mg-Ca-(Cl)-HCO3, Mg-Ca-Na-(Cl)-HCO3 and Mg-Ca-Na-(Cl)-HCO3 type waters. The Na-Cl type waters are originated from deep reservoir during the others can be considered as diluted Cl-HCO3 water type. The plot of delta O-18 versus delta D shows that the geothermal waters are enriched in delta O-18 and dD and located on the mixing trend between groundwaters and seawaters indicating mixing of these both different waters. The proportion of seawaters in geothermal waters seems to be very higher than groundwaters. The shift in the delta O-18 values are related to delta O-18 exchange between the deeply circulating meteoric waters and reservoir rocks in the area. The increase of delta D is related to the contribution of seawaters. The geothermal waters in the area fall into fields of immature to partially equilibrated waters. In general, the reservoir temperature of the area of Cesme is estimated to be 80 to 120 degrees C.