The main high and low enthalpy geothermal fields in the Buyuk Menderes graben (Western Anatolia) and their reservoir temperatures are as follows: Kizildere (242 degreesC), Germencik (232 degreesC), Aydin-Ilicabasi (101 degreesC), Yilmazkoy (142 degreesC), Salavatli (171 degreesC), Soke (26 degreesC), Denizli -Pamukkale (36 degreesC), Karahayit (59 degreesC), Golemezli (101 degreesC) and Yenice (70 degreesC). The geothermal systems are controlled by active graben faults. The reservoir rocks in the geothermal fields are the limestone and conglomerate units within Neogene sediments and the marble-quartzite units within Paleozoic metamorphic formations. There are clear delta(18)O shifts from the Mediterranean Meteoric Water Line (MMWL) in the Kizildere, Germencik and Aydin fields, where a good relation between high temperatures and delta(18)O shift has also been observed, indicating deep circulation and water rock interactions. In the Pamukkale, Karahayit, Golemezli and Yenice fields and in Soke region, low temperatures, small isotope shifts, shallow circulations and mixing with shallow cold water have been noted. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.