The study area, located in the eastern part of the continental rift zone of the Büyük Menderes within the Menderes Massif, western Anatolia, is composed of Paleozoic metamorphic rocks, Mesozoic limestones and Eocene via Pliocene to Quaternary sediments. Paleozoic marbles, quartzites and carbonate schists, Mesozoic limestone, Pliocene sediments and Quaternary alluvium and travertine serve as permeable rocks for the geothermal waters. The geothermal waters in Pamukkale and environs with outlet temperatures of about 35 °C and reservoir temperatures up to 250 °C can be considered as Ca-Mg-SO4-HCO3 type waters. The formation of the travertine in Pamukkale is one of the world’s wonders, directly connected with decreasing temperatures and CO2 partial pressures. The formation of travertine deposits depends upon the solubility of CaCO3 controlled principally by CO2 partial pressure, temperature and pH values, in which reaction equilibriums play an important role. Moreover, the travertine deposits, whic show a U-series age of at least 400 ka form one of the important world wonders. The geothermal waters of Pamukkale, with its high sulphate contents up to 650 mg/l and Rn concentrations up to 20 Bq/l, were modelled hydrogeologically from schematical points.